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Document and Entity Information - shares	9 Months Ended
Sep. 30, 2018	Nov. 02, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q3
Trading Symbol	RGNX
Entity Registrant Name	REGENXBIO Inc.
Entity Central Index Key	0001590877
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		35,840,359

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 142,423	$ 46,656
Marketable securities	230,166	114,122
Accounts receivable	2,626	473
Prepaid expenses	6,308	5,334
Other current assets	4,834	1,412
Total current assets	386,357	167,997
Marketable securities	102,296	15,616
Accounts receivable	4,600
Property and equipment, net	19,856	13,977
Restricted cash	225	225
Other assets	1,650	862
Total assets	514,984	198,677
Current liabilities
Accounts payable	5,214	4,832
Accrued expenses and other current liabilities	13,306	9,605
Deferred revenue	600
Total current liabilities	19,120	14,437
Deferred rent, net of current portion	1,116	1,211
Other liabilities	691
Total liabilities	20,927	15,648
Commitments and contingencies (Note 6)
Stockholders’ equity
Preferred stock; $0.0001 par value; 10,000 shares authorized, and no shares issued and outstanding at September 30, 2018 and December 31, 2017
Common stock; $0.0001 par value; 100,000 shares authorized at September 30, 2018 and December 31, 2017; 35,704 and 31,295 shares issued and outstanding at September 30, 2018 and December 31, 2017, respectively	4	3
Additional paid-in capital	582,249	371,497
Accumulated other comprehensive loss	(874)	(715)
Accumulated deficit	(87,322)	(187,756)
Total stockholders’ equity	494,057	183,029
Total liabilities and stockholders’ equity	$ 514,984	$ 198,677

Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues
Revenues	$ 5,306	$ 1,336	$ 177,728	$ 8,353
Costs of revenues
Research and development	18,508	12,518	59,544	43,054
General and administrative	9,008	9,444	25,706	22,421
Other operating expenses (income)	(2)		31	74
Total operating expenses	28,031	22,645	92,078	67,640
Income (loss) from operations	(22,725)	(21,309)	85,650	(59,287)
Other Income
Interest income from licensing	109		8,362
Investment income	2,122	603	4,177	2,115
Total other income	2,231	603	12,539	2,115
Income (loss) before income taxes	(20,494)	(20,706)	98,189	(57,172)
Income Tax Benefit (Expense)	1,292		(2,558)
Net income (loss)	(19,202)	(20,706)	95,631	(57,172)
Other Comprehensive Income (Loss)
Unrealized gain (loss) on available-for-sale securities, net of reclassifications and income tax expense	(103)	93	(159)	(521)
Total other comprehensive income (loss)	(103)	93	(159)	(521)
Comprehensive income (loss)	(19,305)	(20,613)	95,472	(57,693)
Net income (loss) applicable to common stockholders	$ (19,202)	$ (20,706)	$ 95,631	$ (57,172)
Net income (loss) per share:
Basic	$ (0.56)	$ (0.67)	$ 2.94	$ (1.94)
Diluted	$ (0.56)	$ (0.67)	$ 2.67	$ (1.94)
Weighted-average common shares outstanding:
Basic	33,988	30,940	32,576	29,440
Diluted	33,988	30,940	35,875	29,440
License [Member]
Revenues
Revenues	$ 5,306	$ 1,335	$ 177,728	$ 8,345
Costs of revenues
Costs of revenues	$ 517	683	$ 6,797	2,085
Other [Member]
Revenues
Revenues		$ 1		8
Costs of revenues
Costs of revenues				$ 6

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities
Net income (loss)	$ 95,631	$ (57,172)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities
Stock-based compensation expense	11,755	7,809
Net amortization of premiums and accretion of discounts on marketable debt securities	889	1,410
Depreciation and amortization	2,671	1,963
Net realized losses (gains) on sales of marketable securities	15	(479)
Imputed interest income from licensing	(8,362)
Non-cash consideration received for licenses granted		(420)
Other non-cash adjustments	12	39
Changes in operating assets and liabilities
Accounts receivable	6,986	150
Prepaid expenses	(974)	(1,450)
Other current assets	(3,072)	(380)
Other assets	(788)	(109)
Accounts payable	156	1,142
Accrued expenses and other current liabilities	3,438	3,449
Deferred revenue	600
Deferred rent	(41)	(118)
Other liabilities	(128)
Net cash provided by (used in) operating activities	108,788	(44,166)
Cash flows from investing activities
Purchases of marketable securities	(319,897)	(46,593)
Maturities of marketable securities	116,110	47,225
Sales of marketable securities		780
Purchases of property and equipment	(8,389)	(5,059)
Net cash used in investing activities	(212,176)	(3,647)
Cash flows from financing activities
Proceeds from exercise of stock options	9,051	752
Proceeds from issuance of common stock under employee stock purchase plan	850	556
Proceeds from public offering of common stock, net of underwriting discounts and commissions	189,716	81,994
Issuance costs for public offering of common stock	(462)	(445)
Net cash provided by financing activities	199,155	82,857
Net increase in cash and cash equivalents and restricted cash	95,767	35,044
Cash and cash equivalents and restricted cash
Beginning of period	46,881	25,065
End of period	142,648	60,109
Supplemental disclosures of non-cash investing and financing activities
Purchases of property and equipment in accounts payable and accrued expenses	174
Non-cash consideration received for licenses granted		$ 420
Issuance costs for public offering of common stock in accounts payable and accrued expenses	$ 157

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation and Principles of Consolidation

The accompanying consolidated financial statements are unaudited and have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (ASC) and Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB). The interim unaudited consolidated financial statements have been prepared on the same basis as the annual audited consolidated financial statements as of and for the year ended December 31, 2017 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017, which was filed with the SEC on March 6, 2018. Certain information and footnote disclosures required by GAAP which are normally included in the Company’s annual consolidated financial statements have been omitted pursuant to SEC rules and regulations for interim reporting. In the opinion of management, the accompanying consolidated financial statements reflect all adjustments, which include all normal and recurring adjustments necessary for the fair statement of the Company’s financial position as of September 30, 2018, and the results of its operations and its cash flows for the interim periods ended September 30, 2018 and 2017.

The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year, any other interim periods, or any future year or period. These interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements as of and for the year ended December 31, 2017, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K.

The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could differ materially from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these consolidated financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the consolidated financial statements. Estimates are used in the following areas, among others: revenue, stock-based compensation expense, accrued research and development expenses and other accrued expenses, income taxes and the fair value of financial instruments.

Accounts Receivable

Accounts receivable primarily consist of consideration due to the Company resulting from its license agreements with NAV Technology Licensees. Accounts receivable include amounts invoiced to licensees as well as rights to consideration which have not yet been invoiced to licensees and for which payment is conditional solely upon the passage of time. If a licensee elects to terminate a license prior to the end of the license term, the licensed intellectual property is returned to the Company and any accounts receivable from the licensee which are not contractually payable to the Company are charged off as a reduction of license revenue in the period of the termination. Accounts receivable from licensees which are not expected to be received by the Company within 12 months from the reporting date are stated net of a discount to present value and recorded as non-current assets on the consolidated balance sheets.

Receivables are stated net of an allowance for doubtful accounts, if deemed necessary based on the Company’s evaluation of collectability using specific identification of account balances, the credit profile of its customers and historical information regarding write-offs. Account balances are charged off against the allowance when the potential for recovery is considered remote. The Company did not record an allowance for doubtful accounts as of September 30, 2018 or December 31, 2017.

Non-marketable Equity Securities

The Company’s non-marketable equity securities do not have readily determinable fair values and consist of equity investments in other entities in which the Company’s ownership interest is below 20% and the Company does not have significant influence over the operations of the entity. Prior to January 1, 2018, non-marketable equity securities were accounted for using the cost method and measured at cost less impairment. Beginning January 1, 2018, upon the Company’s adoption of ASU 2016-01, Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, non-marketable equity securities are measured at cost less impairment, adjusted for observable price changes for identical or similar investments of the same issuer. Please refer to Note 4 for further information on non-marketable equity securities.

Fair Value of Financial Instruments

The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. FASB ASC Topic 820, Fair Value Measurements and Disclosures (ASC 820), establishes a hierarchy of inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the observable inputs be used when available. Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances. The fair value hierarchy applies only to the valuation inputs used in determining the reported fair value of the investments and is not a measure of the investment credit quality. The three levels of the fair value hierarchy are described below:

•

Level 1—Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date.

•

Level 2—Valuations based on quoted prices for similar assets or liabilities in markets that are not active or for which all significant inputs are observable, either directly or indirectly.

•

Level 3—Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and unobservable.

To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair values of the Company’s Level 2 instruments are based on quoted market prices or broker or dealer quotations for similar assets. These investments are initially valued at the transaction price and subsequently valued utilizing third party pricing providers or other market observable data. Please refer to Note 4 for further information on the fair value measurement of the Company’s financial instruments.

Revenue Recognition

Effective January 1, 2018, the Company adopted ASU 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes the revenue recognition requirements in ASC 605, Revenue Recognition (Topic 605). Please refer to Recent Accounting Pronouncements below for additional information on the adoption of Topic 606 and the impact upon adoption to the Company’s financial position and results of operations.

Topic 606 requires entities to recognize revenue when control of the promised goods or services is transferred to customers at an amount that reflects the consideration to which the entity expects to be entitled to in exchange for those goods or services. The following five steps are performed to determine the appropriate revenue recognition for arrangements within the scope of Topic 606: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract and (v) recognize revenue when (or as) the entity satisfies the performance obligations.

The Company applies the five-step model to contracts that are within the scope of Topic 606 only when it is probable that the Company will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, for contracts within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determined those that are performance obligations and whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to respective performance obligations when (or as) the respective performance obligations are satisfied.

The Company evaluates its contracts for the presence of significant financing components. If a significant financing component is identified in a contract and provides a financing benefit to the customer, the transaction price for the contract is adjusted to account for the financing portion of the arrangement, which is recognized as interest income over the financing term using the effective interest method. In determining the appropriate interest rates for significant financing components, the Company evaluates the credit profile of the customer and prevailing market interest rates and selects an interest rate in which it believes would be charged to the customer in a separate financing arrangement over a similar financing term.

License revenue

The Company licenses its NAV Technology Platform to other biotechnology and pharmaceutical companies. The terms of the licenses vary, and licenses may be exclusive or non-exclusive and may be sublicensable by the licensee. Licenses may grant intellectual property rights for purposes of internal and preclinical research and development only, or may include the rights, or options to obtain future rights, to commercialize drug therapies for specific diseases using the Company’s NAV Technology Platform. License agreements generally have a term at least equal to the life of the underlying patents, but are terminable at the option of the licensee. Consideration to the Company under its license agreements may include: (i) up-front fees, (ii) option fees to obtain additional licenses, (iii) annual maintenance fees, (iv) milestone payments based on the achievement of certain development and sales-based milestones by licensees, (v) sublicense fees and (vi) royalties on sales of licensed products.

The Company has determined that all of its license agreements are contracts with customers within the scope of Topic 606. Although licenses are terminable at the option of licensee, the Company has determined that there is a substantive termination penalty associated with the termination of each license. Due to the substantive termination penalty, the contract term for purposes of applying Topic 606 is equal to the stated term of the license agreement, which is the life of the underlying licensed patents. The Company’s performance obligations under its license agreements include the delivery of intellectual property licenses to licensees as well as options granted to licensees to acquire future licenses to the extent the options represent material rights to the licensee. The transaction price for each license agreement is allocated to these performance obligations and recognized as revenue when the performance obligations are satisfied. Consideration allocated to performance obligations for the delivery of intellectual property licenses is recognized as revenue upon the delivery of the license(s) to the licensee, which generally occurs upon the execution of the license agreement. Consideration allocated to performance obligations for license options is recognized as revenue upon the earlier of the option exercise or expiration.

For license agreements which contain options for the licensee to purchase additional licenses in the future, the Company evaluates the options at the inception of the agreement to determine if they provide a material right to the licensee. In making this determination, the Company considers whether the optional licenses are priced at a discount to the standalone selling price for the licenses. For options granted which are deemed to be material rights to the licensee, the Company allocates a portion of the transaction price to the performance obligation for the option and recognizes that consideration as revenue at the earlier of option exercise or expiration. Options which are not material rights to licensees are not considered performance obligations and are not accounted for as part of the license agreement until exercised by the licensee. Consideration contingent upon the exercise of options by licensees is excluded from the transaction price and not accounted for as part of the license agreement until the option is exercised. Upon the exercise of an option by a licensee, the additional consideration related to the option exercise is added to the transaction price and recognized as revenue upon the delivery of the newly purchased license.

The Company evaluates the transaction price for its license agreements at each reporting date. The transaction price for each license includes all fixed consideration, as well as variable consideration to the extent that it is probable that a significant reversal of revenue will not occur in the future. Fixed consideration under the Company’s license agreements includes up-front fees and annual maintenance fees. Variable consideration under the Company’s license agreements includes development and sales-based milestone payments, sublicense fees and royalties on sales of licensed products.

Up-front license fees are included in the transaction price and recognized as revenue upon the delivery of the license. If up-front license fees are payable to Company in periods beyond 12 months from the delivery of the license, a significant financing component is deemed to exist and the Company adjusts the transaction price to include only the present value of the license fees. The discounted portion of the license fees is recognized as interest income in the consolidated statements of operations over the term of the financing period.

Annual maintenance fees are generally payable to the Company on each anniversary date over the term of the license agreement. The Company has determined that the payment of annual maintenance fees by licensees in future periods represents a significant financing component to the license since the delivery of the license occurs at the inception of the agreement. The present value of aggregate annual maintenance fees payable to the Company over the term of the license is included in the transaction price and recognized as revenue upon the delivery of the license. The discounted portion of the annual maintenance fees is recognized as interest income in the consolidated statements of operations over the term of the license.

Development milestone payments are payable to the Company upon the achievement of specified development milestones by licensees. At the inception of each license agreement that contains development milestone payments, the Company evaluates whether the milestones are considered probable of achievement and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal will not occur in the future, milestone payments are included in the transaction price and recognized as revenue upon the delivery of the license. Milestone payments contingent on the achievement of development milestones that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved and are excluded from the transaction price until the milestone is achieved. At each reporting date, the Company re-evaluates the probability of achievement of outstanding development milestones and, if necessary, adjusts the transaction price for any milestones for which the probability of achievement has changed due to current facts and circumstances. Any such adjustments are recorded on a cumulative catch-up basis and recorded as license revenue in the period of the adjustment.

Royalties on sales of licensed products, sales-based milestone payments and sublicense fees based on the receipt of certain fees by licensees from any sublicensees are excluded from the transaction price for each license and recognized as revenue in the period that the related sales or sublicenses occur, provided that the associated license has been delivered to the licensee. To date the Company has not recognized any revenue from royalties on sales of licensed products, the achievement of sales-based milestones or sublicense fees.

The Company receives payments from licensees based on the billing schedules established in each license agreement. Amounts recognized as revenue which have not yet been received from licensees are recorded as accounts receivable when the Company’s rights to the consideration are conditional solely upon the passage of time. Amounts recognized as revenue which have not yet been received from licensees are recorded as contract assets when the Company’s rights to the consideration are not unconditional. Contract assets are recorded as other current assets on the consolidated balance sheets. If a licensee elects to terminate a license prior to the end of the license term, the licensed intellectual property is returned to the Company and any consideration recorded as accounts receivable or contract assets which is not contractually payable by the licensee is charged off as a reduction of license revenue in the period of the termination. Amounts received by the Company prior to the delivery of underlying performance obligations are deferred and recognized as revenue upon the satisfaction of the performance obligations by the Company.

Costs of Revenues

Licensing costs consist of sublicense fees incurred by the Company to its licensors as a result of license revenues generated by the Company. Sublicense fees are based on a percentage of license fees received by the Company from licensees as specified in the Company’s agreements with its licensors. The Company recognizes sublicense fees in the period that the underlying license revenue is recognized. Sublicense fees payable by the Company to licensors in periods beyond 12 months from the reporting are recorded as non-current liabilities on the consolidated balance sheets.

Stock-based Compensation

The Company accounts for its stock-based compensation awards in accordance with FASB ASC 718, Compensation—Stock Compensation (ASC 718). ASC 718 requires all stock-based payments to employees and nonemployees, including stock options and restricted stock units, to be recognized in the consolidated statement of operations and comprehensive loss based on their fair values.

The Company’s stock-based awards are subject to either service or performance-based vesting conditions. Compensation expense related to awards to employees and nonemployees with service-based vesting conditions is recognized on a straight-line basis based on the estimated grant date fair value over the associate service period of the award, which is generally the vesting term. Compensation expense related to awards to employees and nonemployees with performance-based vesting conditions is recognized based on the estimated grant date fair value over the requisite service period using the accelerated attribution method to the extent achievement of the performance condition is probable.

The Company estimates the fair value of its stock option awards to employees and nonemployees using the Black-Scholes option-pricing model, which requires the input of subjective assumptions, including (i) the fair value of the underlying common stock, (ii) the expected stock price volatility, (iii) the expected term of the award, (iv) the risk-free interest rate and (v) expected dividends. Due to the lack of Company-specific historical and implied volatility data of its common stock, the Company has based its estimate of expected volatility on its own historical volatility as well as the historical volatility of a group of similar companies that are publicly traded. Due to the lack of historical trading data for its common stock, the Company places a higher weight on the historical volatility of the selected peer group in estimating historical volatility. When selecting these public companies on which it has based its expected stock price volatility, the Company selects companies with comparable characteristics to it, including enterprise value, risk profiles and position within the industry and with historical share price information sufficient to meet the expected term of the Company’s stock-based awards. The Company computes historical volatility data using the daily closing prices for the selected companies’ shares during the equivalent period of the calculated expected term of the stock-based awards. The Company will continue to apply this method of estimating expected volatility until a sufficient amount of historical information regarding the volatility of its own common stock price becomes available. The Company estimates the expected term of its employee stock options using the “simplified” method, whereby, the expected term equals the arithmetic average of the vesting term and the original contractual term of the option due to its lack of sufficient historical data. For stock options granted to nonemployees, the Company uses the contractual term of the award rather than expected term to estimate the fair value of the award. The Company estimates the risk-free interest rates for periods within the expected term of its options based on the rates of U.S. Treasury securities with maturity dates commensurate with the expected term of the associated awards. The Company has never paid and does not expect to pay dividends in the foreseeable future.

The Company estimates the fair value of restricted stock units based on the fair value of the Company’s common stock on the date of the grant.

On July 1, 2018, the Company adopted ASU 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-based Payment Accounting. Prior to the adoption of this standard, compensation expense for the Company’s stock-based awards to nonemployees was based on the then-current fair value of the awards at each reporting date prior to the measurement date, which is generally the vesting date. Upon the adoption of ASU 2018-07, these awards will no longer be remeasured and any new stock-based awards granted to nonemployees after the adoption of the new standard will be measured at the estimated grant date fair value of the awards.

Net Income (Loss) Per Share

Basic net income (loss) per share is calculated by dividing net income (loss) applicable to common stockholders by the weighted-average common shares outstanding during the period, without consideration for common stock equivalents. Diluted net income (loss) per share is calculated by adjusting the weighted-average common shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury-stock method. Contingently convertible shares in which conversion is based on non-market-priced contingencies are excluded from the calculations of both basic and diluted net income (loss) per share until the contingency has been fully met. For purposes of the diluted net income (loss) per share calculation, common stock equivalents are excluded from the calculation of diluted net income (loss) per share if their effect would be anti-dilutive.

Recent Accounting Pronouncements

Adoption of ASU 2014-09, Revenue from Contracts with Customers

In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes the revenue recognition requirements in ASC 605, Revenue Recognition (Topic 605). Effective January 1, 2018, the Company adopted Topic 606 using the modified retrospective transition method. Under this method, the Company applied Topic 606 to all contracts with customers which were not completed as of January 1, 2018 and recorded the cumulative impact of the adoption as an adjustment to its accumulated deficit on January 1, 2018. The Company’s financial results for periods ending after January 1, 2018 are presented in accordance with the requirements of Topic 606, while prior period amounts have not been adjusted and continue to be reported in accordance with Topic 605. Please refer to Revenue Recognition above for additional information on Topic 606, including a description of the Company’s revenue recognition policies upon adoption.

The Company recorded a net reduction in opening accumulated deficit of $4.8 million as of January 1, 2018 for the cumulative impact of adoption of Topic 606, which was primarily the result of accelerated recognition of license revenue due to annual maintenance fees under Topic 606. Under Topic 605, annual maintenance fees payable to the Company by licensees were recognized as license revenue annually when the amounts became fixed or determinable. Under Topic 606, the present value of aggregate annual maintenance fees over the term of the license agreement are recognized as revenue upon the delivery of the license to the licensee. The impact of the accelerated recognition of license revenue upon adoption was partially offset by the accelerated recognition of licensing costs to the Company’s licensors. The Company recognizes sublicense fees to its licensors in the period the underlying license revenue is recognized.

The cumulative adjustment for the adoption of Topic 606 had the following effects on the Company’s consolidated balance sheet as of January 1, 2018 (in thousands):

				Cumulative
				Adjustment for
	Balance at			Adoption of		Balance at
	December 31, 2017			Topic 606		January 1, 2018
Consolidated Balance Sheet
Assets:
Accounts receivable, current	$	473		$	527	$	1,000
Accounts receivable, non-current	$	—		$	4,850	$	4,850
Other current assets	$	1,412		$	350	$	1,762
Liabilities:
Accrued expenses and other current liabilities	$	9,605		$	105	$	9,710
Other liabilities	$	—		$	819	$	819
Stockholdersʼ Equity:
Accumulated deficit	$	(187,756	)	$	4,803	$	(182,953	)

The following tables present the effects of the adoption of Topic 606 on each financial statement line item of the Company’s financial statements for the interim periods ended September 30, 2018 (in thousands, except per share data):

	As of September 30, 2018
				Impact of		Results Without
				Adoption of		Adoption of
	As Reported			Topic 606		Topic 606
Consolidated Balance Sheet
Assets:
Accounts receivable, current	$	2,626		$	626	$	2,000
Accounts receivable, non-current	$	4,600		$	4,600	$	—
Prepaid expenses	$	6,308		$	60	$	6,248
Other current assets	$	4,834		$	2,000	$	2,834
Liabilities:
Accrued expenses and other current liabilities	$	13,306		$	322	$	12,984
Deferred revenue	$	600		$	600	$	—
Other liabilities	$	691		$	691	$	—
Stockholdersʼ Equity:
Accumulated deficit	$	(87,322	)	$	5,673	$	(92,995	)

	Three Months Ended September 30, 2018
				Impact of		Results Without
				Adoption of		Adoption of
	As Reported			Topic 606		Topic 606
Consolidated Statement of Operations
Revenues:
License revenue	$	5,306		$	2,066	$	3,240
Expenses:
Licensing costs	$	517		$	193	$	324
Other Income:
Interest income from licensing	$	109		$	109	$	—
Net Income (Loss)	$	(19,202	)	$	1,982	$	(21,184	)
Net Income (Loss) Per Share:
Basic	$	(0.56	)	$	0.06	$	(0.62	)
Diluted	$	(0.56	)	$	0.06	$	(0.62	)

	Nine Months Ended September 30, 2018
			Impact of			Results Without
			Adoption of			Adoption of
	As Reported		Topic 606			Topic 606
Consolidated Statement of Operations
Revenues:
License revenue	$	177,728	$	(7,462	)	$	185,190
Expenses:
Licensing costs	$	6,797	$	30		$	6,767
Other Income:
Interest income from licensing	$	8,362	$	8,362		$	—
Net Income (Loss)	$	95,631	$	870		$	94,761
Net Income (Loss) Per Share:
Basic	$	2.94	$	0.03		$	2.91
Diluted	$	2.67	$	0.03		$	2.64

	Nine Months Ended September 30, 2018
				Impact of			Results Without
				Adoption of			Adoption of
	As Reported			Topic 606			Topic 606
Consolidated Statement of Cash Flows
Cash Flows from Operating Activities:
Net income	$	95,631		$	870		$	94,761
Imputed interest income from licensing	$	(8,362	)	$	(8,362	)	$	—
Changes in accounts receivable	$	6,986		$	8,513		$	(1,527	)
Changes in prepaid expenses	$	(974	)	$	(60	)	$	(914	)
Changes in other current assets	$	(3,072	)	$	(1,650	)	$	(1,422	)
Changes in accrued expenses and other current liabilities	$	3,438		$	217		$	3,221
Changes in deferred revenue	$	600		$	600		$	—
Changes in other liabilities	$	(128	)	$	(128	)	$	—

The most significant effects that the adoption of Topic 606 had on the results of operations for the three and nine months ended September 30, 2018, as compared to what results would have been if Topic 605 had continued to be applied, is the amount of (i) license revenue recognized related to licensee development milestones and (ii) interest income from licensing recognized as a result of significant financing components identified within the Company’s license agreements. Under Topic 606, development milestone payments from licensees are recognized as revenue to the extent that the development milestone is probable of achievement by the licensee and the underlying license has been delivered to the licensee by the Company. Under Topic 605, development milestone payments, if substantive, were not recognized as revenue until the development milestone was achieved by the licensee. During the three and nine months ended September 30, 2018, the Company recognized $2.0 million of license revenue, which would not have been recognized under the requirements of Topic 605, related to development milestones which were not achieved as of September 30, 2018 but were deemed probable of achievement by licensees and recorded as contract assets within other current assets on the Company’s consolidated balance sheet. Additionally, under Topic 606, if a significant financing component is identified within a license agreement, the Company is required to adjust the amount of revenue recognized upon the delivery of the license to the present value of the underlying consideration. The discounted portion of the consideration is recognized as interest income from licensing over the financing term of the license agreement. Under Topic 605, the amount of revenue recognized from the delivery of licenses is not adjusted for significant financing components. During the three and nine months ended September 30, 2018, the Company recognized $0.1 million and $8.4 million, respectively, of interest income from licensing as a result of significant financing components identified in its license agreements. The Company would not have recognized any interest income from significant financing components during these periods under the requirements of Topic 605.

Other recently adopted accounting pronouncements

In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-based Payment Accounting which supersedes the existing guidance for accounting for stock-based awards to nonemployees under ASC 505-50, Equity—Equity-based Payments to Nonemployees. The new guidance expands the scope of Topic 718 to include stock-based awards to nonemployees for goods or services. Consequently, the accounting for stock-based awards to employees and nonemployees will be substantially aligned. The standard is effective for annual and interim periods beginning after December 15, 2018, with early adoption permitted upon issuance. The Company elected to early adopt this standard effective July 1, 2018, which required the Company to remeasure all of its outstanding stock-based awards to nonemployees which do not yet have established measurement dates to the estimated fair value on the adoption date. These awards, which consisted solely of stock options granted to third-party advisors with performance-based vesting conditions, will no longer be remeasured and any new stock-based awards granted to nonemployees after the adoption of the new standard will be measured at the estimated grant date fair value of the awards. The new standard requires the cumulative effect of the adoption on the adoption date to be presented as an adjustment to retained earnings as of the beginning of the year of adoption. On the adoption date, the Company had no unvested and outstanding stock-based awards to nonemployees which were probable of vesting. Accordingly, the adoption of this standard required no retrospective adjustment and did not have a material impact on the Company’s financial position or results of operations.

In May 2017, the FASB issued ASU 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting, which clarifies when modification accounting should be applied for changes to terms or conditions of a stock-based award. The standard is effective for annual and interim periods beginning after December 15, 2017, with early adoption permitted, and is to be applied prospectively upon adoption. The Company adopted this standard effective January 1, 2018. The adoption of this standard did not have a material impact on the Company’s financial position or results of operations.

In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash. The standard requires that the statement of cash flows explain the change during the period in the total of cash, cash equivalents and restricted cash. As a result, amounts generally described as restricted cash should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. The standard is effective for annual and interim periods beginning after December 15, 2017, with early adoption permitted, and is to be applied retrospectively to each period presented. The Company adopted this standard effective January 1, 2018. The adoption of this standard did not have a material impact on the Company’s consolidated statements of cash flows.

The Company’s restricted cash consists of money market mutual funds used to collateralize an irrevocable letter of credit as required by the Company’s lease agreement for its office space in New York, New York. The following table provides a reconciliation of cash and cash equivalents and restricted cash as reported on the consolidated balance sheets to the total of these amounts as reported at the end of the period in the consolidated statements of cash flows (in thousands):

	September 30, 2018		September 30, 2017
Cash and cash equivalents	$	142,423	$	59,884
Restricted cash		225		225
Total cash and cash equivalents and restricted cash	$	142,648	$	60,109

In January 2016, the FASB issued ASU 2016-01, Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, which modifies the current guidance on the recognition, measurement, presentation and disclosure of financial instruments. In February 2018, the FASB issued ASU 2018-03, Technical Corrections and Improvements to Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, which clarifies the guidance in ASU 2016-01. The Company adopted these standards effective January 1, 2018. Upon the adoption of these standards, the Company elected to measure its non-marketable equity securities without readily available fair values at cost less impairment, adjusted for observable price changes for identical or similar investments of the same issuer. Prior to the adoption of these standards, the Company measured these investments at cost less impairment. The adoption of these standards did not have a material impact on the Company’s financial position or results of operations.

Recent accounting pronouncements not yet adopted

In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement, which modifies certain disclosure requirements regarding fair value measurements. The standard is effective for annual and interim periods beginning after December 15, 2019, with early adoption permitted upon issuance. The Company does not believe the application of this standard will have a material impact on the Company’s disclosures.

In February 2018, the FASB issued ASU 2018-02, Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income which amends the current guidance on comprehensive income to provide an option for an entity to reclassify the stranded tax effects of the Tax Cuts and Jobs Act of 2017 (the TCJA) that was signed into law in December 2017 from accumulated other comprehensive income directly to retained earnings. The stranded tax effects result from the remeasurement of deferred tax assets and liabilities which were originally recorded in comprehensive income but whose remeasurement is reflected in the income statement. The standard is effective for annual and interim periods beginning after December 15, 2018, with early adoption permitted upon issuance. The Company is evaluating the application of this standard but has not yet determined the potential effects it may have on the Company’s consolidated financial statements.

In April 2017, the FASB issued ASU 2017-08, Receivables—Nonrefundable Fees and Other Costs (Subtopic 310-20), which amends the required amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period for the premium to the earliest call date. The standard is effective for annual and interim periods beginning after December 15, 2019, with early adoption permitted upon issuance, and is to be applied on a modified retrospective basis through a cumulative-effect adjustment directly to retained earnings as of the beginning of the period of adoption. The Company does not believe the application of this standard will have a material impact on the Company’s financial position or results of operations.

In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which amends the accounting for credit losses for most financial assets and certain other instruments. The standard requires that entities holding financial assets and net investment in leases that are not accounted for at fair value through net income be presented at the net amount expected to be collected. An allowance for credit losses will be a valuation account that will be deducted from the amortized cost basis of the financial asset to present the net carrying value at the amount expected to be collected on the financial asset. The standard is effective for annual and interim periods beginning after December 15, 2019, with early adoption permitted for annual and interim periods beginning after December 15, 2018. The Company does not believe the application of this standard will have a material impact on the Company’s financial position or results of operations.

In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) which supersedes FASB ASC Topic 840, Leases (Topic 840) and provides principles for the recognition, measurement, presentation and disclosure of leases for both lessees and lessors. The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease, respectively. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases. In July 2018, the FASB issued ASU 2018-10, Codification Improvements to Topic 842, Leases and ASU 2018-11, Leases (Topic 842): Targeted Improvements which clarify and simplify multiple aspects of the guidance under Topic 842. The standards are effective for annual and interim periods beginning after December 15, 2018, with early adoption permitted upon issuance. The Company plans to adopt these standards using the modified retrospective transition method and will apply to leases in effect as of, or entered into after, January 1, 2019. The Company believes the new standards will significantly impact the accounting for its operating leases for office and laboratory facilities as well as any embedded leases within the Company’s service contracts. The Company expects the adoption of these standards will have a material impact on the Company’s consolidated balance sheet, as they will require the Company to recognize a right-of-use asset and lease liability for each of its lease agreements within the scope of the new standard. However, the Company does not expect the adoption of these standards to have a material impact on the Company’s results of operations.

Marketable Securities

9 Months Ended

Sep. 30, 2018

Investments Debt And Equity Securities [Abstract]

Marketable Securities

3. Marketable Securities

The following tables present a summary of the Company’s marketable securities, which consist solely of available-for-sale debt securities (in thousands):

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
September 30, 2018
U.S. government and federal agency securities	$	68,437	$	—	$	(145	)	$	68,292
Certificates of deposit		6,806		—		—			6,806
Corporate bonds		257,658		5		(299	)		257,364
	$	332,901	$	5	$	(444	)	$	332,462

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
December 31, 2017
Corporate bonds	$	130,018	$	2	$	(282	)	$	129,738
	$	130,018	$	2	$	(282	)	$	129,738

As of September 30, 2018 and December 31, 2017, no available-for-sale securities had remaining maturities greater than three years.

The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. As of September 30, 2018 and December 31, 2017, the balance in the Company’s accumulated other comprehensive loss consisted solely of net unrealized gains and losses on available-for-sale securities, net of income tax effects and reclassification adjustments for realized gains and losses. During the three months and nine months ended September 30, 2018, the Company recognized net unrealized losses on available-for-sale securities of $0.1 million and $0.2 million, respectively, and income tax expense of $0 in other comprehensive income (loss) for the period. The Company recognized net realized losses of less than $0.1 million on the sale or maturity of available-for-sale securities during the three and nine months ended September 30, 2018, which were reclassified out of accumulated other comprehensive loss during the period and are included in investment income in the consolidated statements of operations and comprehensive income (loss). During the three and nine months ended September 30, 2017, the Company recognized net unrealized gains (losses) on available-for-sale securities of $0.1 million and less than $(0.1) million, respectively, and income tax expense of $0 in other comprehensive income (loss) for the period. The Company recognized net realized gains of less than $0.1 million and $0.5 million on the sale or maturity of marketable securities during the three and nine months ended September 30, 2017, respectively, which were reclassified out of accumulated other comprehensive loss during the period and are included in investment income in the consolidated statements of operations and comprehensive income (loss).

The following tables present the fair values and unrealized losses of marketable securities held by the Company in an unrealized loss position for less than 12 months and 12 months or greater (in thousands):

	Less than 12 Months					12 Months or Greater					Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
September 30, 2018
U.S. government and federal agency securities	$	68,292	$	(145	)	$	—	$	—		$	68,292	$	(145	)
Corporate bonds		222,129		(259	)		10,588		(40	)		232,717		(299	)
	$	290,421	$	(404	)	$	10,588	$	(40	)	$	301,009	$	(444	)

	Less than 12 Months					12 Months or Greater					Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
December 31, 2017
Corporate bonds	$	109,238	$	(180	)	$	17,124	$	(102	)	$	126,362	$	(282	)
	$	109,238	$	(180	)	$	17,124	$	(102	)	$	126,362	$	(282	)

As of September 30, 2018, securities held by the Company which were in an unrealized loss position consisted of 106 investment grade fixed income security positions. The Company has the intent and ability to hold such securities until recovery and has determined that none of its investments were other-than-temporarily impaired as of September 30, 2018 or December 31, 2017.

Fair Value of Financial Instruments

9 Months Ended

Sep. 30, 2018

Fair Value Disclosures [Abstract]

Fair Value of Financial Instruments

4. Fair Value of Financial Instruments

Financial instruments reported at fair value on a recurring basis include cash equivalents and marketable securities. The following tables present the fair value of cash equivalents and marketable securities in accordance with the hierarchy discussed in Note 2 (in thousands):

	Quoted		Significant
	prices		other		Significant
	in active		observable		unobservable
	markets		inputs		inputs
	(Level 1)		(Level 2)		(Level 3)		Total
September 30, 2018
Cash equivalents:
Money market mutual funds	$	—	$	136,761	$	—	$	136,761
Corporate bonds		—		5,644		—		5,644
Total cash equivalents		—		142,405		—		142,405
Marketable securities:
Certificates of deposit		—		6,806		—		6,806
U.S. government and federal agency securities		—		68,292		—		68,292
Corporate bonds		—		257,364		—		257,364
Total marketable securities		—		332,462		—		332,462
Total cash equivalents and marketable securities	$	—	$	474,867	$	—	$	474,867

	Quoted		Significant
	prices		other		Significant
	in active		observable		unobservable
	markets		inputs		inputs
	(Level 1)		(Level 2)		(Level 3)		Total
December 31, 2017
Cash equivalents:
Money market mutual funds	$	—	$	46,646	$	—	$	46,646
Total cash equivalents		—		46,646		—		46,646
Marketable securities:
Corporate bonds		—		129,738		—		129,738
Total marketable securities		—		129,738		—		129,738
Total cash equivalents and marketable securities	$	—	$	176,384	$	—	$	176,384

There were no transfers of financial instruments between levels of the fair value hierarchy during the nine months ended September 30, 2018.

Management estimates that the carrying amounts of its current accounts receivable, accounts payable and accrued expenses and other current liabilities approximate fair value due to the short-term nature of those instruments. Non-current accounts receivable are recorded at their present values using a discount rate that is based on prevailing market rates and the credit profile of the licensee on the date the amounts are initially recorded. Management does not believe there have been any significant changes in market conditions that would cause the discount rates used to be significantly different from those that would be used as of September 30, 2018 to determine the present value of the receivables. Accordingly, management estimates that the carrying value of its non-current accounts receivable approximates the fair value of those instruments.

The Company’s non-marketable equity securities are measured at cost less impairment, adjusted for observable price changes for identical or similar investments of the same issuer. As of September 30, 2018 and December 31, 2017, non-marketable equity securities had carrying values of $0.4 million and are included in other assets on the consolidated balance sheets. Since the acquisition of the securities, the Company has not identified any observable price changes or changes in circumstances that would have an adverse effect on the fair value of its non-marketable equity securities as of September 30, 2018. No impairment losses on non-marketable equity securities were recorded during the three and nine months ended September 30, 2018 and 2017.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Commitments and Contingencies

9 Months Ended

Sep. 30, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

6. Commitments and Contingencies

Lease Agreements

The Company recognizes rent expense on a straight-line basis over the term of its operating leases commencing on the date the Company takes possession of the leased property. Tenant improvement allowances that are considered to be lease incentives from the lessor are recorded as deferred rent and amortized as a reduction of rent expense over the term of the lease from the possession date.

In March 2015, the Company entered into a non-cancelable operating lease for office space at 9712 Medical Center Drive in Rockville, Maryland (the 9712 Medical Center Drive Lease). The lease term commenced in April 2015. Monthly payments under the lease began in October 2015 and escalate annually in accordance with the lease agreement.

In September 2015, November 2015, July 2017 and April 2018, the Company amended the 9712 Medical Center Drive Lease to include additional office and laboratory space at 9714 Medical Center Drive, and ultimately extend the term of the lease to September 2021. The Company has options to extend the term of the 9712 Medical Center Drive Lease for up to six additional years. Under the amended lease, the Company has received a $0.4 million tenant improvement allowance from the landlord which will be deferred and amortized on a straight-line basis as a reduction of rent expense over the term of the lease.

In January 2016, the Company entered into a 7.5-year, non-cancelable operating lease for its corporate headquarters at 9600 Blackwell Road in Rockville, Maryland (the Blackwell Road Lease). The lease commenced in February 2016, and expires in September 2023. The Company has an option to extend the term of the lease for an additional five years. In November 2017, the Blackwell Road Lease was amended to include additional office space for the remainder of the lease term. Monthly payments under the lease began in September 2016 and escalate annually in accordance with the lease agreement. The Company received a $0.8 million tenant improvement allowance from the landlord which will be deferred and amortized on a straight-line basis as a reduction of rent expense over the term of lease.

In May 2016, the Company entered into a 51-month, non-cancelable operating lease for additional office space at 400 Madison Avenue in New York, New York. The lease commenced in July 2016, and expires in October 2020. Monthly payments under the lease began in October 2016 and escalate annually in accordance with the lease agreement. Under the terms of the lease agreement, the Company has provided the landlord with an irrevocable letter of credit of $0.2 million which the landlord may draw upon in the event of any uncured default by the Company under the terms of the lease. As of September 30, 2018, the Company has recorded restricted cash of $0.2 million as collateral to the financial institution which issued the letter of credit.

In September 2018, the Company entered into a 39-month, non-cancelable operating lease for additional office and laboratory space at 9600 Medical Center Drive in Rockville, Maryland. The lease commenced in October 2018, and expires in December 2021. Monthly payments under the lease begin in December 2018 and escalate annually in accordance with the lease agreement.

As of September 30, 2018, future minimum lease payments under non-cancelable operating leases are as follows (in thousands):

	Operating
	Leases
2018 (remainder of year)	$	660
2019		2,946
2020		2,975
2021		2,335
2022		621
Thereafter		479
Total minimum lease payments	$	10,016

In November 2018, the Company entered into an operating lease for approximately 132,000 square feet of office and laboratory facilities in a new building to be constructed at 9800 Medical Center Drive in Rockville, Maryland (the 9800 Medical Center Drive Lease). Construction of the new building, which will be conducted by the landlord, is expected to be completed in mid-2020 and the lease will expire approximately 16 years from the delivery of the leased premises to the Company, subject to certain extension and termination options. Under the terms of the 9800 Medical Center Drive Lease, the Company will receive a $14.6 million tenant improvement allowance from the landlord to construct additional improvements to the leased premises. The Company has the option to extend the term of the lease for up to 10 additional years and the option to terminate the lease after 12 years from the delivery of the leased premises to the Company. If the Company elects to terminate the lease, it will be subject to a termination fee equal to the unamortized tenant improvement allowance, rent abatement and landlord commissions as of the termination date, bearing interest at 5% per annum, plus four months of base rent and operating expenses. Additionally, after delivery of the leased premises under the 9800 Medical Center Drive Lease, the Company will have the option to terminate the 9712 Medical Center Drive Lease with six months’ notice. Monthly payments under the 9800 Medical Center Drive Lease begin approximately 12 months from the delivery of the leased premises to the Company and escalate annually in accordance with the lease agreement. Total future minimum lease payments under the 9800 Medical Center Drive Lease, which are subject to adjustment based on the actual square footage of the facility to be constructed, are estimated to be $87.6 million and payments are expected to begin in 2021.

Licenses Granted to the Company

The Company licenses intellectual property from third-parties, primarily for technology used in the Company’s product candidates and development programs and which may be sublicensed to NAV Technology Licensees. Licenses granted to the Company may require the Company to make future payments relating to sublicense fees, milestone fees for milestones achieved in the future and royalties on future sales of licensed products. Additionally, the Company may be responsible for the cost of the maintenance of the intellectual property as incurred by its licensors. Up-front fees to obtain licensed technology are included in research and development expenses and patent maintenance costs are included in general and administrative expenses in the consolidated statements of operations and comprehensive income (loss). Sublicense fees are based on a specified percentage of license fees earned by the Company and are included in licensing costs in the consolidated statements of operations and comprehensive income (loss). Milestone fees are included in licensing costs in the consolidated statements of operations and comprehensive loss if the underlying milestone is achieved by a licensee, or in research and development expense if the underlying milestone is achieved by the Company as a result of the development of its product candidates. Royalties on sales of licensed reagents for use in research and development are included in other costs of revenue in the consolidated statements of operations and comprehensive income (loss). The Company has not commercialized any product candidates or paid any royalties under these agreements other than for the sales of licensed reagents.

The Trustees of the University of Pennsylvania. In February 2009, the Company entered into a license agreement, which has been amended from time to time, with The Trustees of the University of Pennsylvania (together with the University of Pennsylvania, Penn) for exclusive, worldwide rights to certain patents owned by Penn underlying the Company’s NAV Technology Platform. Under the terms of the agreement, in consideration for the license, the Company issued to Penn a 24.5% equity interest in the Company on a fully diluted basis after issuance. The Company is obligated to pay Penn royalties on net sales and sublicense fees, if any. Additionally, the Company is obligated to reimburse Penn for certain costs incurred related to the maintenance of the licensed patents.

In April 2016, the Company entered into an agreement with Penn whereby the Company will fund clinical trial activities performed by Penn for RGX-501, the Company’s product candidate for the treatment of homozygous familial hypercholesterolemia (HoFH). In connection with the agreement, the Company amended its license from Penn to include exclusive license rights to data, results and other information generated in connection with the RGX-501 clinical trial.

Expenses incurred by the Company related to its license from Penn were as follows (in thousands):

	Three Months Ended September 30,					Nine Months Ended September 30,
	2018			2017		2018			2017
Sublicense fees	$	(14	)	$	92	$	(14	)	$	793
Royalties on sales of reagents		—			—		—			4
Maintenance of licensed patents		—			28		117			197
	$	(14	)	$	120	$	103		$	994

As of September 30, 2018 and December 31, 2017, the Company had accrued $0.1 million and less than $0.1 million, respectively, in expenses payable to Penn under the license agreement, which are included in accounts payable, accrued expenses and other current liabilities and other liabilities on the Company’s consolidated balance sheets.

GlaxoSmithKline LLC. In March 2009, the Company entered into a license agreement, which was amended in April 2009, with GlaxoSmithKline LLC (GSK) for exclusive, worldwide rights to certain patents underlying the Company’s NAV Technology Platform which are owned by Penn and exclusively licensed to GSK. Under the terms of the agreement, in consideration for the license, the Company issued to GSK a 19.9% equity interest in the Company on a fully diluted basis after issuance. The Company is obligated to pay GSK royalties on net sales and sublicense fees, if any. Additionally, the Company is obligated to reimburse GSK for certain costs incurred and invoiced to the Company related to the maintenance of the licensed patents. The Company is also obligated to pay GSK up to $1.5 million upon the achievement of various milestones. From the inception of the agreement through September 30, 2018, the Company has incurred $0.5 million for milestones that have been achieved or are deemed probable of achievement.

Expenses incurred by the Company related to its license from GSK were as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Sublicense fees	$	531	$	92	$	6,561	$	793
Royalties on sales of reagents		—		—		—		2
Milestone fees		—		500		—		500
Maintenance of licensed patents		163		290		653		449
	$	694	$	882	$	7,214	$	1,744

As of September 30, 2018 and December 31, 2017, the Company had accrued $1.7 million and $0.3 million, respectively, in expenses payable to GSK under the license agreement, which are included in accounts payable, accrued expenses and other current liabilities and other liabilities on the Company’s consolidated balance sheets.

Other Licenses

In November 2014, the Company entered into a license agreement, which was amended in November 2016, with Regents of the University of Minnesota (Minnesota), for an exclusive license under Minnesota’s interest in certain patent rights which are co-owned by Minnesota and the Company to commercialize products covered by the licensed patent rights in any country or territory in which a licensed patent has been issued and is unexpired, or a licensed patent application is pending. In consideration for the license, the Company paid an up-front fee, and reimbursed Minnesota for patent maintenance expenses incurred to date. Under the terms of the agreement, the Company is obligated to pay Minnesota annual maintenance fees, royalties on net sales and sublicense fees, if any, and fees upon the achievement of various milestones. In November 2016, the license with Minnesota was amended to include additional patent rights. In consideration for the additional patent rights, the Company paid an up-front fee.

In August 2018, the Company entered into a license agreement with Emory University (Emory) for an exclusive license under Emory’s interest in certain patent rights which are co-owned by Emory and the Company to commercialize products covered by the licensed patent rights in any country or territory in which a licensed patent has been issued and is unexpired, or a licensed patent application is pending. In consideration for the license, the Company paid an up-front fee and is obligated to reimburse Emory for patent prosecution and maintenance expenses and pay Emory annual maintenance fees under certain circumstances, royalties on net sales and sublicense fees, if any, and fees upon the achievement of various milestones for the first licensed product.

Other Funding Commitments

In the normal course of business, the Company enters into agreements with contract research organizations, contract manufacturing organizations and other third-parties for services to be provided to the Company. Generally, these agreements provide for termination upon notice, with specified amounts due upon termination based on the timing of termination and the terms of the agreement. The actual amounts and timing of payments under these agreements are uncertain and contingent upon the initiation and completion of services to be provided to the Company.

Guarantees and Indemnifications

In the normal course of business, the Company enters into agreements that contain a variety of representations and provide for general indemnification. The Company’s potential exposure under these agreements is unknown because it involves claims that may be made against the Company in the future. To date, the Company has not paid any claims or been required to defend any action related to its indemnification obligations. As of September 30, 2018 and December 31, 2017, the Company did not have any material indemnification claims that were probable or reasonably possible and consequently has not recorded any related liabilities.

European Patent Office Proceeding

In June 2017, a third party filed an opposition with the European Patent Office (EPO) challenging the validity of a European patent owned by Penn for the AAV8 vector, which the Company has exclusively licensed (EU AAV8 Patent). The EPO conducted oral proceedings in October 2018, and upheld the validity of the EU AAV8 Patent subject to certain amendments made during the proceeding. As of September 30, 2018, the Company has not recorded any liabilities related to this matter.

License Revenue

9 Months Ended

Sep. 30, 2018

License Agreement Revenue Recognition [Abstract]

License Revenue

7. License Revenue

Effective January 1, 2018, the Company adopted Topic 606 using the modified retrospective transition method and has applied the new standard to all of its license agreements in effect as of January 1, 2018. Please refer to Note 2 for additional information regarding the adoption of Topic 606. License revenue for periods ending after January 1, 2018 is presented in accordance with the requirements of Topic 606, while prior period amounts have not been adjusted and continue to be reported in accordance with Topic 605 and accordingly, may not be comparable.

As of September 30, 2018, the Company’s NAV Technology Platform was being applied in the development of more than 20 partnered product candidates by its NAV Technology Licensees. Consideration to the Company under its license agreements may include: (i) up-front fees, (ii) option fees to obtain additional licenses, (iii) annual maintenance fees, (iv) milestone payments based on the achievement of certain development and sales-based milestones by licensees, (v) sublicense fees and (vi) royalties on sales of licensed products. Sublicense fees vary by license and range from a mid-single digit percentage to a low-double digit percentage of license fees received by licensees as a result of sublicenses. Royalties on net sales of commercialized products vary by license and range from a mid-single digit percentage to a low double-digit percentage of net sales by licensees. To date the Company has not recognized any revenue from the achievement of sales-based milestones, royalties on sales of licensed products or sublicense fees.

Development milestone payments are only included in the transaction price of each license and recognized as revenue to the extent they are considered probable of achievement. Sales-based milestones are excluded from the transaction price of each license agreement and recognized as revenue in the period of achievement. As of September 30, 2018, the Company’s license agreements, excluding additional licenses that could be granted upon the exercise of options by licensees, contained unachieved development milestones which could result in aggregate milestone fees payable to the Company of up to $27.4 million upon the commencement of various stages of clinical trials, $45.5 million upon the submission of regulatory approval filings, $111.5 million upon the approval of commercial products by regulatory agencies and $172.0 million upon the achievement of specified sales targets for licensed products. To the extent the milestone fees are realized by the Company, the Company will be obligated to pay sublicense fees to licensors based on a specified percentage of the fees earned by the Company. The achievement of milestones by licensees is highly dependent on the successful development and commercialization of licensed products and it is at least reasonably possible that some or all of the milestone fees will not be realized by the Company.

The following table presents changes in the balances of the Company’s receivables, contract assets and contract liabilities during the periods presented (in thousands):

	Balance at
	Beginning		Net Additions		Balance at
	of Period		(Deductions)		End of Period
Three Months Ended September 30, 2018
Receivables and contract assets:
Accounts receivable, current and non-current	$	5,224	$	2,002	$	7,226
Contract assets	$	—	$	2,000	$	2,000
Contract liabilities:
Deferred revenue	$	600	$	—	$	600

	Balance at
	Beginning		Net Additions		Balance at
	of Period		(Deductions)		End of Period
Nine Months Ended September 30, 2018
Receivables and contract assets:
Accounts receivable, current and non-current	$	5,850	$	1,376	$	7,226
Contract assets	$	350	$	1,650	$	2,000
Contract liabilities:
Deferred revenue	$	—	$	600	$	600

The net increase in the balance of accounts receivable during the three and nine months ended September 30, 2018 is primarily attributable to the achievement of a regulatory development milestone by a licensee in September 2018. The Company recognized license revenue of $2.0 million upon the achievement of the milestone, which was previously not considered probable of achievement. The net increase in the balance of contract assets during the three and nine months ended September 30, 2018 is primarily attributable to a regulatory development milestone which was deemed probable of achievement by a licensee during the period but was not achieved as of September 30, 2018. The Company recognized license revenue of $2.0 million during the period related to the milestone which was previously not considered probable of achievement.

As of September 30, 2018, the Company had recorded deferred revenue of $0.6 million which represents consideration received from licensees for performance obligations that have not yet been satisfied by the Company. Unsatisfied performance obligations consist of options granted to licensees that provide material rights to the licensee to acquire future licenses from the Company. These performance obligations will be satisfied, and underlying revenue will be recognized, upon the exercise or expiration of the options.

During the three and nine months ended September 30, 2018, the Company recognized license revenue of $4.1 million and $4.2 million, respectively, from licenses delivered to licensees in prior periods as a result of changes in the transaction prices of its license agreements. Changes in the transaction price were primarily attributable to development milestones achieved or deemed probable of achievement during the period that were previously not considered probable of achievement. Additionally, the Company recognized $0.1 million and $0.4 million of interest income from licensing during the three and nine months ended September 30, 2018, respectively, from licenses delivered in prior periods which contained significant financing components.

As of September 30, 2018, the Company had not recognized any impairment losses on its receivables or contract assets from contracts with customers.

AveXis, Inc. March 2014 License and January 2018 Amendment

In March 2014, the Company entered into an exclusive license agreement (the March 2014 License) with AveXis, Inc. (AveXis). Under the license, the Company granted AveXis an exclusive, worldwide commercial license, with rights to sublicense, to the NAV AAV9 vector for the treatment of spinal muscular atrophy (SMA) in humans by in vivo gene therapy. In consideration for the license, AveXis paid the Company an up-front fee of $2.0 million, and is required to pay annual maintenance fees, development milestone payments of up to $12.3 million, mid-single to low double-digit royalties on net sales of licensed products, subject to reduction in specified circumstances, and a lower mid-double digit percentage of any sublicense fees AveXis receives from sublicensees for the licensed intellectual property rights.

In January 2018, the Company entered into an amendment (the January 2018 Amendment) to the March 2014 License with AveXis. Under the January 2018 Amendment, the licensed intellectual property was expanded to include, in addition to the NAV AAV9 vector previously licensed, any other recombinant AAV vector in the Company’s intellectual property portfolio during a period of 14 years from the effective date of the January 2018 Amendment, for the treatment of SMA in humans by in vivo gene therapy. The Company may also, in its sole discretion, provide specified collaborative services to AveXis as specified in the January 2018 Amendment.

The January 2018 Amendment also modified the terms and conditions of the March 2014 License relating to assignment. Under the amended assignment provision, AveXis is permitted to transfer the March 2014 License, as amended, without the Company’s consent in connection with a change of control of AveXis, subject to the transferee or successor agreeing in writing to be bound by the terms of the March 2014 License, as amended, and the payment to the Company of certain fees due upon such change of control, as described below. Under the original March 2014 License, any assignment by AveXis without the Company’s prior written consent had been prohibited.

Pursuant to the January 2018 Amendment, in consideration for the additional rights granted thereunder and in addition to any consideration owed under the original March 2014 License, AveXis paid to the Company a fee of $80.0 million upon entry into the January 2018 Amendment. In addition, AveXis was obligated to pay the Company (i) $30.0 million on the first anniversary of the effective date of the January 2018 Amendment, (ii) $30.0 million on the second anniversary of the effective date of the January 2018 Amendment and (iii) potential sales-based milestone payments of up to $120.0 million. In the event of a change of control of AveXis, to the extent that any fee described in (i) or (ii) above, or the first $40.0 million of sales-based milestone payments described in (iii) above, had not yet been paid to the Company, the January 2018 Amendment obliged AveXis to pay any such unpaid fee to the Company upon the change of control. For any product developed for the treatment of SMA using the NAV AAV9 vector, AveXis will continue to be obligated to pay to the Company mid-single to low double-digit royalties on net sales as defined in the March 2014 License, and for any product developed for the treatment of SMA using a licensed vector other than NAV AAV9, the Company will receive a low double-digit royalty on net sales.

In May 2018, AveXis was acquired by Novartis AG (Novartis), which qualified as a change of control of AveXis under the January 2018 Amendment. Pursuant to the January 2018 Amendment, AveXis paid the Company $100.0 million in accelerated license payments as a result of the change of control.

Accounting Analysis

The January 2018 Amendment was accounted for under Topic 606 as a modification of the license agreement resulting in a new and separate contract from the original March 2014 License for revenue recognition purposes. The only material performance obligation of the Company under the January 2018 Amendment is for the delivery of the modified license, which occurred upon the execution of the amendment in January 2018.

As of September 30, 2018, the transaction price of the original March 2014 License was $7.5 million. The transaction price of $7.5 million includes (i) the up-front payment in March 2014 of $2.0 million, (ii) the present value of aggregate annual maintenance fees payable to the Company over the term of the license and (iii) the development milestones that had been achieved to date or are deemed probable of achievement. The discounted portion of the annual maintenance fees represents the financing benefit provided to AveXis and is recognized as interest income from licensing over the term of the license. Variable consideration under the original March 2014 License, which has been excluded from the transaction price, includes payments for remaining development milestones that have not yet been achieved and are not considered probable of achievement, as well as any potential sublicense fees or royalties on sales of licensed products, which will be recognized in the period of the underlying sales or sublicenses, if any.

Upon its execution, the transaction price of the January 2018 Amendment was $132.1 million, which was fully recognized as license revenue upon the delivery of the modified license in January 2018. In May 2018, as a result of the acquisition of AveXis by Novartis, the transaction price was increased by $40.0 million to account for the acceleration of the sale-based milestone which was previously excluded from the transaction price. The $40.0 million increase in the transaction price was recognized as license revenue upon the completion of the change of control in May 2018 since the amended license had been fully delivered to AveXis. Additionally, due to the acceleration of the two $30.0 million payments originally due in January 2019 and January 2020, the Company recognized $6.1 million of interest income from licensing upon the completion of the change of control of AveXis, which represents the remaining present value discount on such payments as of the date of the change of control of AveXis. The transaction price of $172.1 million as of September 30, 2018 includes the following fixed consideration: (i) the $80.0 million payment in January 2018, (ii) the present value, as of the date of the January 2018 Amendment, of the two $30.0 million payments originally due in January 2019 and January 2020 and (iii) the $40.0 million sales-based milestone which was accelerated upon the change of control in May 2018. Variable consideration under the January 2018 Amendment, which has been excluded from the transaction price, includes the remaining sales-based milestone payment of $80.0 million, as well as any potential sublicense fees or royalties on sales of licensed products, which will be recognized in the period of the underlying sales or sublicenses, if any.

During the three and nine months ended September 30, 2018, the Company recognized license revenue of $4.0 million and $176.1 million, respectively, and interest income from licensing of less than $0.1 million and $8.0 million, respectively, from the March 2014 License, as amended, with AveXis, which includes the amounts from both the original March 2014 License and the January 2018 Amendment. As of September 30, 2018, the Company had recorded $2.2 million of accounts receivable from AveXis under the March 2014 License, as amended, of which $2.0 million are included in current assets and $0.2 million are included in non-current assets on the consolidated balance sheets. As of September 30, 2018, the Company had recorded $2.0 million of contract assets for development milestones deemed probable of achievement by AveXis under the March 2014 License, as amended, which are included in other current assets on the consolidated balance sheets.

During the three and nine months ended September 30, 2017, the Company recognized license revenue of $0 and $0.1 million, respectively, from the March 2014 License which was recognized under the requirements of Topic 605. As of December 31, 2017, the Company had no amounts receivable from AveXis related to the March 2014 License under the requirements of Topic 605.

AveXis, Inc. June 2017 License

In June 2017, the Company entered into an exclusive license agreement (the June 2017 License) with AveXis. Under the license, the Company granted AveXis an exclusive, worldwide commercial license, with rights to sublicense, to the NAV AAV9 vector for the treatment of Rett Syndrome and amyotrophic lateral sclerosis (ALS) caused by mutations in the gene that produces the copper zinc superoxide dismutase 1 (SOD1) in humans by in vivo gene therapy. In consideration for the license, AveXis paid the Company an up-front fee of $6.0 million, and is required to pay annual maintenance fees, development milestone payments of up to $36.0 million, a low double-digit royalty percentage on net sales of licensed products, subject to reduction in specified circumstances, and a lower mid-double digit percentage of any sublicense fees AveXis receives from sublicensees for the licensed intellectual property rights.

During the three and nine months ended September 30, 2018, the Company recognized license revenue of $0 and interest income from licensing of less than $0.1 million and $0.1 million, respectively, from the June 2017 License with AveXis. As of September 30, 2018, the Company had recorded $0.8 million of accounts receivable from AveXis under the June 2017 License, of which $0.1 million are included in current assets and $0.7 million are included in non-current assets on the consolidated balance sheets.

During the three and nine months ended September 30, 2017, the Company recognized license revenue of $0 and $6.0 million, respectively, from the June 2017 License which was recognized under the requirements of Topic 605. As of December 31, 2017, the Company had no amounts receivable from AveXis related to the June 2017 License under the requirements of Topic 605.

Abeona Therapeutics Inc.

In November 2018, the Company entered into an exclusive license agreement with Abeona Therapeutics Inc. (Abeona). Under the license, the Company granted Abeona an exclusive worldwide commercial license (subject to certain non-exclusive rights previously granted by the Company), with rights to sublicense, to the NAV AAV9 vector for the treatment of Mucopolysaccharidosis Type IIIA (MPS IIIA), also known as Sanfilippo Syndrome Type A, Mucopolysaccharidosis Type IIIB (MPS IIIB), also known as Sanfilippo Syndrome Type B, Neuronal Ceroid Lipfuscinosis-1 (CLN1 disease), also known as infantile Batten Disease, and Neuronal Ceroid Lipfuscinosis-3 (CLN3 disease), also known as juvenile Batten Disease, by in vivo gene therapy. In consideration for the license, Abeona is obligated to pay up-front and annual payments to the Company totaling up to $120.0 million, which are payable as follows: (i) $10.0 million payable upon the execution of the license agreement, (ii) $10.0 million payable within 12 months of the effective date of the license agreement, and (iii) $100.0 million payable in five annual installments of $20.0 million beginning on the second anniversary of the effective date of the license agreement, which are subject to reduction should Abeona terminate some but not all of the licensed indications. Any unpaid portion of the first $40.0 million of up-front and annual payments described above shall become payable upon termination of the license agreement. In the event of a change of control of Abeona, any remaining unpaid up-front and annual payments described above shall become payable upon the change of control. As additional consideration for the license, Abeona is also obligated to pay the Company up to $60.0 million upon the achievement of specified sales-based milestones, low double-digit royalties on net sales of licensed products, subject to reduction in specified circumstances, and a lower mid-double digit percentage of any sublicense fees Abeona receives from sublicensees for the licensed intellectual property rights.

Stock-based Compensation

9 Months Ended

Sep. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-based Compensation

8. Stock-based Compensation

In January 2018, an additional 1,251,810 shares became available for issuance under the 2015 Equity Incentive Plan (the 2015 Plan). As of September 30, 2018, the total number of shares of common stock authorized for issuance under the 2015 Plan and 2014 Stock Plan (the 2014 Plan) was 9,488,413, of which 2,196,366 remain available for future grants under the 2015 Plan.

Stock-based Compensation Expense

The Company’s stock-based compensation expense by award type is as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Stock options	$	4,309	$	2,598	$	11,248	$	7,370
Restricted stock units		69		69		206		207
Employee stock purchase plan		105		67		301		232
	$	4,483	$	2,734	$	11,755	$	7,809

As of September 30, 2018, the Company had $40.8 million of unrecognized stock-based compensation expense related to stock options, restricted stock units and the 2015 Employee Stock Purchase Plan (the 2015 ESPP), which is expected to be recognized over a weighted-average period of 2.7 years.

The Company has recorded aggregate stock-based compensation expense in the consolidated statements of operations and comprehensive income (loss) as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Research and development	$	1,940	$	1,424	$	5,364	$	3,799
General and administrative		2,543		1,310		6,391		4,010
	$	4,483	$	2,734	$	11,755	$	7,809

Stock Options

The following table summarizes stock option activity under the 2014 Plan and 2015 Plan (in thousands, except per share data):

						Weighted-
						average
				Weighted-		Remaining
				average		Contractual		Aggregate
				Exercise		Life		Intrinsic
	Shares			Price		(Years)		Value (a)
Outstanding at December 31, 2017		5,468		$	10.25		7.9	$	125,738
Granted		1,275		$	38.37
Exercised		(1,275	)	$	7.22
Cancelled or forfeited		(297	)	$	16.91
Outstanding at September 30, 2018		5,171		$	17.55		7.7	$	299,646
Exercisable at September 30, 2018		2,726		$	8.85		6.9	$	181,718
Vested and expected to vest at September 30, 2018		5,150		$	17.62		7.7	$	298,060

(a)	The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at the dates reported.

The weighted-average grant date fair value per share of options granted during the nine months ended September 30, 2018 was $25.57. During the nine months ended September 30, 2018, the total number of stock options exercised was 1,274,980, resulting in total proceeds of $9.1 million. The total intrinsic value of options exercised during the nine months ended September 30, 2018 was $47.5 million.

Restricted Stock Units

The following table summarizes restricted stock unit activity under the 2015 Plan (in thousands, except per share data):

			Weighted-
			average
			Grant Date
	Shares		Fair Value
Unvested balance at December 31, 2017		40	$	20.90
Granted		—	$	—
Vested		—	$	—
Forfeited		—	$	—
Unvested balance at September 30, 2018		40	$	20.90

Employee Stock Purchase Plan

As of September 30, 2018, the total number of shares of common stock authorized for issuance under the 2015 ESPP was 254,000, of which 169,580 remain available for future issuance. During the nine months ended September 30, 2018, 36,700 shares of common stock were issued under the 2015 ESPP.

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2018

Accounting Policies [Abstract]

Basis of Presentation and Principles of Consolidation

Use of Estimates

Accounts Receivable

Non-marketable Equity Securities

Fair Value of Financial Instruments

•

Level 1—Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date.

•

Level 2—Valuations based on quoted prices for similar assets or liabilities in markets that are not active or for which all significant inputs are observable, either directly or indirectly.

•

Level 3—Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and unobservable.

Revenue Recognition

License revenue

Cost of Revenues

Costs of Revenues

Stock-based Compensation

The Company estimates the fair value of restricted stock units based on the fair value of the Company’s common stock on the date of the grant.

Net Income (Loss) Per Share

Recent Accounting Pronouncements

Adoption of ASU 2014-09, Revenue from Contracts with Customers

The cumulative adjustment for the adoption of Topic 606 had the following effects on the Company’s consolidated balance sheet as of January 1, 2018 (in thousands):

				Cumulative
				Adjustment for
	Balance at			Adoption of		Balance at
	December 31, 2017			Topic 606		January 1, 2018
Consolidated Balance Sheet
Assets:
Accounts receivable, current	$	473		$	527	$	1,000
Accounts receivable, non-current	$	—		$	4,850	$	4,850
Other current assets	$	1,412		$	350	$	1,762
Liabilities:
Accrued expenses and other current liabilities	$	9,605		$	105	$	9,710
Other liabilities	$	—		$	819	$	819
Stockholdersʼ Equity:
Accumulated deficit	$	(187,756	)	$	4,803	$	(182,953	)

	As of September 30, 2018
				Impact of		Results Without
				Adoption of		Adoption of
	As Reported			Topic 606		Topic 606
Consolidated Balance Sheet
Assets:
Accounts receivable, current	$	2,626		$	626	$	2,000
Accounts receivable, non-current	$	4,600		$	4,600	$	—
Prepaid expenses	$	6,308		$	60	$	6,248
Other current assets	$	4,834		$	2,000	$	2,834
Liabilities:
Accrued expenses and other current liabilities	$	13,306		$	322	$	12,984
Deferred revenue	$	600		$	600	$	—
Other liabilities	$	691		$	691	$	—
Stockholdersʼ Equity:
Accumulated deficit	$	(87,322	)	$	5,673	$	(92,995	)

	Three Months Ended September 30, 2018
				Impact of		Results Without
				Adoption of		Adoption of
	As Reported			Topic 606		Topic 606
Consolidated Statement of Operations
Revenues:
License revenue	$	5,306		$	2,066	$	3,240
Expenses:
Licensing costs	$	517		$	193	$	324
Other Income:
Interest income from licensing	$	109		$	109	$	—
Net Income (Loss)	$	(19,202	)	$	1,982	$	(21,184	)
Net Income (Loss) Per Share:
Basic	$	(0.56	)	$	0.06	$	(0.62	)
Diluted	$	(0.56	)	$	0.06	$	(0.62	)

	Nine Months Ended September 30, 2018
			Impact of			Results Without
			Adoption of			Adoption of
	As Reported		Topic 606			Topic 606
Consolidated Statement of Operations
Revenues:
License revenue	$	177,728	$	(7,462	)	$	185,190
Expenses:
Licensing costs	$	6,797	$	30		$	6,767
Other Income:
Interest income from licensing	$	8,362	$	8,362		$	—
Net Income (Loss)	$	95,631	$	870		$	94,761
Net Income (Loss) Per Share:
Basic	$	2.94	$	0.03		$	2.91
Diluted	$	2.67	$	0.03		$	2.64

	Nine Months Ended September 30, 2018
				Impact of			Results Without
				Adoption of			Adoption of
	As Reported			Topic 606			Topic 606
Consolidated Statement of Cash Flows
Cash Flows from Operating Activities:
Net income	$	95,631		$	870		$	94,761
Imputed interest income from licensing	$	(8,362	)	$	(8,362	)	$	—
Changes in accounts receivable	$	6,986		$	8,513		$	(1,527	)
Changes in prepaid expenses	$	(974	)	$	(60	)	$	(914	)
Changes in other current assets	$	(3,072	)	$	(1,650	)	$	(1,422	)
Changes in accrued expenses and other current liabilities	$	3,438		$	217		$	3,221
Changes in deferred revenue	$	600		$	600		$	—
Changes in other liabilities	$	(128	)	$	(128	)	$	—

Other recently adopted accounting pronouncements

	September 30, 2018		September 30, 2017
Cash and cash equivalents	$	142,423	$	59,884
Restricted cash		225		225
Total cash and cash equivalents and restricted cash	$	142,648	$	60,109

Recent accounting pronouncements not yet adopted

Summary of Significant Accounting Policies (Tables)

9 Months Ended

Sep. 30, 2018

Summary of Reconciliation of Cash and Cash Equivalents and Restricted Cash

	September 30, 2018		September 30, 2017
Cash and cash equivalents	$	142,423	$	59,884
Restricted cash		225		225
Total cash and cash equivalents and restricted cash	$	142,648	$	60,109

ASU 2014-09 [Member]

Schedule of Impact on Financial Statements on Adoption of Topic 606

The cumulative adjustment for the adoption of Topic 606 had the following effects on the Company’s consolidated balance sheet as of January 1, 2018 (in thousands):

				Cumulative
				Adjustment for
	Balance at			Adoption of		Balance at
	December 31, 2017			Topic 606		January 1, 2018
Consolidated Balance Sheet
Assets:
Accounts receivable, current	$	473		$	527	$	1,000
Accounts receivable, non-current	$	—		$	4,850	$	4,850
Other current assets	$	1,412		$	350	$	1,762
Liabilities:
Accrued expenses and other current liabilities	$	9,605		$	105	$	9,710
Other liabilities	$	—		$	819	$	819
Stockholdersʼ Equity:
Accumulated deficit	$	(187,756	)	$	4,803	$	(182,953	)

	As of September 30, 2018
				Impact of		Results Without
				Adoption of		Adoption of
	As Reported			Topic 606		Topic 606
Consolidated Balance Sheet
Assets:
Accounts receivable, current	$	2,626		$	626	$	2,000
Accounts receivable, non-current	$	4,600		$	4,600	$	—
Prepaid expenses	$	6,308		$	60	$	6,248
Other current assets	$	4,834		$	2,000	$	2,834
Liabilities:
Accrued expenses and other current liabilities	$	13,306		$	322	$	12,984
Deferred revenue	$	600		$	600	$	—
Other liabilities	$	691		$	691	$	—
Stockholdersʼ Equity:
Accumulated deficit	$	(87,322	)	$	5,673	$	(92,995	)

	Three Months Ended September 30, 2018
				Impact of		Results Without
				Adoption of		Adoption of
	As Reported			Topic 606		Topic 606
Consolidated Statement of Operations
Revenues:
License revenue	$	5,306		$	2,066	$	3,240
Expenses:
Licensing costs	$	517		$	193	$	324
Other Income:
Interest income from licensing	$	109		$	109	$	—
Net Income (Loss)	$	(19,202	)	$	1,982	$	(21,184	)
Net Income (Loss) Per Share:
Basic	$	(0.56	)	$	0.06	$	(0.62	)
Diluted	$	(0.56	)	$	0.06	$	(0.62	)

	Nine Months Ended September 30, 2018
			Impact of			Results Without
			Adoption of			Adoption of
	As Reported		Topic 606			Topic 606
Consolidated Statement of Operations
Revenues:
License revenue	$	177,728	$	(7,462	)	$	185,190
Expenses:
Licensing costs	$	6,797	$	30		$	6,767
Other Income:
Interest income from licensing	$	8,362	$	8,362		$	—
Net Income (Loss)	$	95,631	$	870		$	94,761
Net Income (Loss) Per Share:
Basic	$	2.94	$	0.03		$	2.91
Diluted	$	2.67	$	0.03		$	2.64

	Nine Months Ended September 30, 2018
				Impact of			Results Without
				Adoption of			Adoption of
	As Reported			Topic 606			Topic 606
Consolidated Statement of Cash Flows
Cash Flows from Operating Activities:
Net income	$	95,631		$	870		$	94,761
Imputed interest income from licensing	$	(8,362	)	$	(8,362	)	$	—
Changes in accounts receivable	$	6,986		$	8,513		$	(1,527	)
Changes in prepaid expenses	$	(974	)	$	(60	)	$	(914	)
Changes in other current assets	$	(3,072	)	$	(1,650	)	$	(1,422	)
Changes in accrued expenses and other current liabilities	$	3,438		$	217		$	3,221
Changes in deferred revenue	$	600		$	600		$	—
Changes in other liabilities	$	(128	)	$	(128	)	$	—

Marketable Securities (Tables)

9 Months Ended

Sep. 30, 2018

Investments Debt And Equity Securities [Abstract]

Summary of Company Marketable Securities

The following tables present a summary of the Company’s marketable securities, which consist solely of available-for-sale debt securities (in thousands):

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
September 30, 2018
U.S. government and federal agency securities	$	68,437	$	—	$	(145	)	$	68,292
Certificates of deposit		6,806		—		—			6,806
Corporate bonds		257,658		5		(299	)		257,364
	$	332,901	$	5	$	(444	)	$	332,462

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
December 31, 2017
Corporate bonds	$	130,018	$	2	$	(282	)	$	129,738
	$	130,018	$	2	$	(282	)	$	129,738

Summary of Fair Values and Unrealized Losses of Marketable Securities Held by the Company in an Unrealized Loss Position for Less Than 12 months and 12 Months or Greater

	Less than 12 Months					12 Months or Greater					Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
September 30, 2018
U.S. government and federal agency securities	$	68,292	$	(145	)	$	—	$	—		$	68,292	$	(145	)
Corporate bonds		222,129		(259	)		10,588		(40	)		232,717		(299	)
	$	290,421	$	(404	)	$	10,588	$	(40	)	$	301,009	$	(444	)

	Less than 12 Months					12 Months or Greater					Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
December 31, 2017
Corporate bonds	$	109,238	$	(180	)	$	17,124	$	(102	)	$	126,362	$	(282	)
	$	109,238	$	(180	)	$	17,124	$	(102	)	$	126,362	$	(282	)

Fair Value of Financial Instruments (Tables)

9 Months Ended

Sep. 30, 2018

Fair Value Disclosures [Abstract]

Schedule of Fair Value of Cash Equivalents and Marketable Securities

The following tables present the fair value of cash equivalents and marketable securities in accordance with the hierarchy discussed in Note 2 (in thousands):

	Quoted		Significant
	prices		other		Significant
	in active		observable		unobservable
	markets		inputs		inputs
	(Level 1)		(Level 2)		(Level 3)		Total
September 30, 2018
Cash equivalents:
Money market mutual funds	$	—	$	136,761	$	—	$	136,761
Corporate bonds		—		5,644		—		5,644
Total cash equivalents		—		142,405		—		142,405
Marketable securities:
Certificates of deposit		—		6,806		—		6,806
U.S. government and federal agency securities		—		68,292		—		68,292
Corporate bonds		—		257,364		—		257,364
Total marketable securities		—		332,462		—		332,462
Total cash equivalents and marketable securities	$	—	$	474,867	$	—	$	474,867

	Quoted		Significant
	prices		other		Significant
	in active		observable		unobservable
	markets		inputs		inputs
	(Level 1)		(Level 2)		(Level 3)		Total
December 31, 2017
Cash equivalents:
Money market mutual funds	$	—	$	46,646	$	—	$	46,646
Total cash equivalents		—		46,646		—		46,646
Marketable securities:
Corporate bonds		—		129,738		—		129,738
Total marketable securities		—		129,738		—		129,738
Total cash equivalents and marketable securities	$	—	$	176,384	$	—	$	176,384

Stock-based Compensation (Tables)

9 Months Ended

Sep. 30, 2018

Stock-Based Compensation Expense by Award Type

The Company’s stock-based compensation expense by award type is as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Stock options	$	4,309	$	2,598	$	11,248	$	7,370
Restricted stock units		69		69		206		207
Employee stock purchase plan		105		67		301		232
	$	4,483	$	2,734	$	11,755	$	7,809

Stock-Based Compensation Expense

The Company has recorded aggregate stock-based compensation expense in the consolidated statements of operations and comprehensive income (loss) as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Research and development	$	1,940	$	1,424	$	5,364	$	3,799
General and administrative		2,543		1,310		6,391		4,010
	$	4,483	$	2,734	$	11,755	$	7,809

Summary of Unvested RSUs Activity Under 2015 Plan

The following table summarizes restricted stock unit activity under the 2015 Plan (in thousands, except per share data):

			Weighted-
			average
			Grant Date
	Shares		Fair Value
Unvested balance at December 31, 2017		40	$	20.90
Granted		—	$	—
Vested		—	$	—
Forfeited		—	$	—
Unvested balance at September 30, 2018		40	$	20.90

2014 and 2015 Equity Incentive Plan [Member]

Summary of Stock Option Activity

The following table summarizes stock option activity under the 2014 Plan and 2015 Plan (in thousands, except per share data):

						Weighted-
						average
				Weighted-		Remaining
				average		Contractual		Aggregate
				Exercise		Life		Intrinsic
	Shares			Price		(Years)		Value (a)
Outstanding at December 31, 2017		5,468		$	10.25		7.9	$	125,738
Granted		1,275		$	38.37
Exercised		(1,275	)	$	7.22
Cancelled or forfeited		(297	)	$	16.91
Outstanding at September 30, 2018		5,171		$	17.55		7.7	$	299,646
Exercisable at September 30, 2018		2,726		$	8.85		6.9	$	181,718
Vested and expected to vest at September 30, 2018		5,150		$	17.62		7.7	$	298,060

(a)	The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at the dates reported.

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended							9 Months Ended	12 Months Ended	115 Months Ended
	Nov. 30, 2018 USD ($) ft²	Sep. 30, 2018 USD ($)	May 31, 2016 USD ($)	Jan. 31, 2016 USD ($)	Sep. 30, 2015 USD ($)	Mar. 31, 2015	Mar. 31, 2009 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Other Commitments [Line Items]
Tenant improvement allowance received					$ 400,000
Restricted cash as collateral with financial institution		$ 225,000						$ 225,000	$ 225,000	$ 225,000	$ 225,000
Future minimum lease payments		10,016,000						10,016,000		10,016,000
Claims paid to date related to indemnification issues								0
Accruals or expenses related to indemnification issues								0	0
European Patent Office Proceeding [Member]
Other Commitments [Line Items]
Liability related to third party opposition		0						$ 0		0
The Trustees of the University of Pennsylvania [Member]
Other Commitments [Line Items]
Percentage of ownership interest issued as consideration for license agreement								24.50%
Accrued expenses		100,000						$ 100,000		100,000
The Trustees of the University of Pennsylvania [Member] \| Maximum [Member]
Other Commitments [Line Items]
Accrued expenses									100,000
GlaxoSmithKline LLC [Member]
Other Commitments [Line Items]
Percentage of ownership interest issued as consideration for license agreement							19.90%
Accrued expenses		$ 1,700,000						$ 1,700,000	$ 300,000	1,700,000
Milestone payment obligation							$ 1,500,000
GlaxoSmithKline LLC [Member] \| Milestone Fees [Member]
Other Commitments [Line Items]
Related party transaction expense										$ 500,000
Rockville, Maryland [Member]
Other Commitments [Line Items]
Lease term commenced date		2018-10		2016-02		2015-04
Lease term								September 2015, November 2015 and July 2017: Ultimately extend the term of the lease to September 2021.
Options to extend the additional lease term					6 years
Tenant improvement allowance received				$ 800,000
Lease period		39 months		7 years 6 months				39 months		39 months
Lease expiration date		Dec. 31, 2021		Sep. 30, 2023
Options to extend the lease				5 years
Monthly lease payments commencement period		2018-12		2016-09
Rockville, Maryland [Member] \| 9800 Medical Center Drive Lease [Member] \| Subsequent Event [Member]
Other Commitments [Line Items]
Number of square feet to be constructed \| ft²	132,000
Construction completion, description	Construction of the new building, which will be conducted by the landlord, is expected to be completed in mid-2020
Lease term, from the delivery of leased premises	16 years
Lease term, description	the lease will expire approximately 16 years from the delivery of the leased premises to the Company, subject to certain extension and termination options. Under the terms of the 9800 Medical Center Drive Lease, the Company will receive a $14.6 million tenant improvement allowance from the landlord to construct additional improvements to the leased premises.
Tenant improvement allowance to be received	$ 14,600,000
Option to extend the lease	true
Additional option to extend the lease, term	10 years
Option to terminate the lease	true
Option to terminate the lease, description	the option to terminate the lease after 12 years from the delivery of the leased premises to the Company. If the Company elects to terminate the lease, it will be subject to a termination fee equal to the unamortized tenant improvement allowance, rent abatement and landlord commissions as of the termination date, bearing interest at 5% per annum, plus four months of base rent and operating expenses. Additionally, after delivery of the leased premises under the 9800 Medical Center Drive Lease, the Company will have the option to terminate the 9712 Medical Center Drive Lease with six months’ notice.
Minimum release term for option to terminate	12 years
Percentage of early termination fee	5.00%
Term of required base rent to be paid as termination fee	4 months
Frequency of periodic payment	Monthly payments under the 9800 Medical Center Drive Lease begin approximately 12 months from the delivery of the leased premises to the Company and escalate annually in accordance with the lease agreement.
Periodic payment term, from the delivery of leased premises	12 months
Future minimum lease payments	$ 87,600,000
Expected lease beginning year	2021
New York, New York [Member]
Other Commitments [Line Items]
Lease term commenced date			2016-07
Lease expiration date			Oct. 31, 2020
Monthly lease payments commencement period			2016-10
Lease expiration term			51 months
Letters of credit			$ 200,000

License Revenue - Additional Information (Detail)	1 Months Ended				3 Months Ended		9 Months Ended
License Revenue - Additional Information (Detail)	May 31, 2018 USD ($)	Jan. 31, 2018 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2014 USD ($)	Sep. 30, 2018 USD ($) ProductCandidate	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) ProductCandidate	Sep. 30, 2017 USD ($)	Nov. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
License Revenue [Line Items]
Milestone fee upon commencement of clinical trials in humans							$ 27,400,000
Milestone fee upon submission of regulatory approval filings							45,500,000
Milestone fee upon approval of commercial products by regulatory agencies							111,500,000
Milestone fee upon achievement of specified sales targets for licensed products							172,000,000
License revenue					$ 5,306,000	$ 1,336,000	177,728,000	$ 8,353,000
Deferred revenue					600,000		600,000			$ 600,000
Interest income from licensing					109,000		8,362,000
Impirment losses on receivables or contract assets							$ 0
Transaction price description							Upon its execution, the transaction price of the January 2018 Amendment was $132.1 million, which was fully recognized as license revenue upon the delivery of the modified license in January 2018. In May 2018, as a result of the acquisition of AveXis by Novartis, the transaction price was increased by $40.0 million to account for the acceleration of the sale-based milestone which was previously excluded from the transaction price. The $40.0 million increase in the transaction price was recognized as license revenue upon the completion of the change of control in May 2018 since the amended license had been fully delivered to AveXis. Additionally, due to the acceleration of the two $30.0 million payments originally due in January 2019 and January 2020, the Company recognized $6.1 million of interest income from licensing upon the completion of the change of control of AveXis, which represents the remaining present value discount on such payments as of the date of the change of control of AveXis. The transaction price of $172.1 million as of September 30, 2018 includes the following fixed consideration: (i) the $80.0 million payment in January 2018, (ii) the present value, as of the date of the January 2018 Amendment, of the two $30.0 million payments originally due in January 2019 and January 2020 and (iii) the $40.0 million sales-based milestone which was accelerated upon the change of control in May 2018. Variable consideration under the January 2018 Amendment, which has been excluded from the transaction price, includes the remaining sales-based milestone payment of $80.0 million, as well as any potential sublicense fees or royalties on sales of licensed products, which will be recognized in the period of the underlying sales or sublicenses, if any.
Accounts receivable, current					2,626,000		$ 2,626,000				$ 473,000
Accounts receivable, non -current					4,600,000		4,600,000
Contract assets					2,000,000		2,000,000				350,000
Abeona Therapeutics Inc [Member] \| Scenario Forecast [Member]
License Revenue [Line Items]
Upfront and annual payments upon termination of license agreement									$ 40,000,000
Sales-based milestone payments									60,000,000
Maximum [Member] \| Abeona Therapeutics Inc [Member] \| Scenario Forecast [Member]
License Revenue [Line Items]
Up-front and anuual payments									120,000,000
License [Member]
License Revenue [Line Items]
License revenue					5,306,000	1,335,000	177,728,000	8,345,000
License [Member] \| Increase In Accounts Receivable [Member]
License Revenue [Line Items]
License revenue					2,000,000		2,000,000
License [Member] \| Increase In Contract Assets [Member]
License Revenue [Line Items]
License revenue					$ 2,000,000		$ 2,000,000
NAV Technology Licensees [Member] \| NAV Technology Platform [Member] \| Minimum [Member]
License Revenue [Line Items]
Number of development partnered product candidates \| ProductCandidate					20		20
Licence Agreement [Member]
License Revenue [Line Items]
Interest income from licensing					$ 100,000		$ 400,000
Licence Agreement [Member] \| License [Member]
License Revenue [Line Items]
License revenue					4,100,000		4,200,000
March 2014 License Agreement [Member]
License Revenue [Line Items]
Up-front fee paid							2,000,000
Transaction price of license					7,500,000		7,500,000
Accounts receivable											0
Contract assets					2,000,000		2,000,000
March 2014 License Agreement [Member] \| AveXis, Inc. [Member]
License Revenue [Line Items]
Interest income from licensing							8,000,000
Up-front fee paid				$ 2,000,000
Milestone fee payments upon achievement of various development and commercialization				$ 12,300,000
License agreement amendment date				2018-01
Accounts receivable					2,200,000		2,200,000
Accounts receivable, current					2,000,000		2,000,000
Accounts receivable, non -current					200,000		200,000
March 2014 License Agreement [Member] \| Maximum [Member] \| AveXis, Inc. [Member]
License Revenue [Line Items]
Interest income from licensing					100,000
March 2014 License Agreement [Member] \| License [Member]
License Revenue [Line Items]
License revenue					0		100,000
March 2014 License Agreement [Member] \| License [Member] \| AveXis, Inc. [Member]
License Revenue [Line Items]
License revenue					4,000,000		176,100,000
January 2018 Amendment Agreement [Member]
License Revenue [Line Items]
Interest income from licensing	$ 6,100,000
License fee		$ 80,000,000
Sales-based milestone payment unpaid in the event of change of control		40,000,000
Transaction price of license		132,100,000			172,100,000		172,100,000
January 2018 Amendment Agreement [Member] \| First Anniversary [Member]
License Revenue [Line Items]
License fee		30,000,000
January 2018 Amendment Agreement [Member] \| Second Anniversary [Member]
License Revenue [Line Items]
License fee		$ 30,000,000
January 2018 Amendment Agreement [Member] \| AveXis, Inc. [Member]
License Revenue [Line Items]
License agreement period		14 years
License fees received	100,000,000
January 2018 Amendment Agreement [Member] \| Minimum [Member]
License Revenue [Line Items]
Milestone payment		$ 120,000,000
January 2018 Amendment Agreement [Member] \| License [Member]
License Revenue [Line Items]
Increase in transaction price of license	$ 40,000,000
June 2017 License Agreement [Member] \| AveXis, Inc. [Member]
License Revenue [Line Items]
Up-front fee paid			$ 6,000,000
Milestone fee payments upon achievement of various development and commercialization			$ 36,000,000
Accounts receivable					800,000		800,000				$ 0
Accounts receivable, current					100,000		100,000
Accounts receivable, non -current					700,000		700,000
June 2017 License Agreement [Member] \| Maximum [Member] \| AveXis, Inc. [Member]
License Revenue [Line Items]
Interest income from licensing					100,000		100,000
June 2017 License Agreement [Member] \| License [Member] \| AveXis, Inc. [Member]
License Revenue [Line Items]
License revenue					$ 0	$ 0	$ 0	$ 6,000,000
November 2018 [Member] \| Abeona Therapeutics Inc [Member] \| Scenario Forecast [Member]
License Revenue [Line Items]
Up-front and anuual payments									10,000,000
November 2019 [Member] \| Abeona Therapeutics Inc [Member] \| Scenario Forecast [Member]
License Revenue [Line Items]
Up-front and anuual payments									10,000,000
November 2020 [Member] \| Abeona Therapeutics Inc [Member] \| Scenario Forecast [Member]
License Revenue [Line Items]
Up-front and anuual payments									20,000,000
November 2021 [Member] \| Abeona Therapeutics Inc [Member] \| Scenario Forecast [Member]
License Revenue [Line Items]
Up-front and anuual payments									20,000,000
November 2022 [Member] \| Abeona Therapeutics Inc [Member] \| Scenario Forecast [Member]
License Revenue [Line Items]
Up-front and anuual payments									20,000,000
November 2023 [Member] \| Abeona Therapeutics Inc [Member] \| Scenario Forecast [Member]
License Revenue [Line Items]
Up-front and anuual payments									20,000,000
November 2024 [Member] \| Abeona Therapeutics Inc [Member] \| Scenario Forecast [Member]
License Revenue [Line Items]
Up-front and anuual payments									$ 20,000,000

Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	35,704,000	31,295,000
Common stock, shares outstanding	35,704,000	31,295,000

Nature of Business	9 Months Ended
Nature of Business	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of Business	1. Nature of Business REGENXBIO Inc. (the Company) is a leading clinical-stage biotechnology company seeking to improve lives through the curative potential of gene therapy. The Company’s proprietary adeno-associated virus (AAV) gene delivery platform (NAV Technology Platform) consists of exclusive rights to over 100 novel AAV vectors, including AAV7, AAV8, AAV9 and AAVrh10. The Company’s NAV® Technology Platform is being applied by the Company, as well as by third-party licensees (NAV Technology Licensees), in the development of product candidates for a variety of diseases with unmet needs. The Company was formed in 2008 in the State of Delaware and is headquartered in Rockville, Maryland. Liquidity and Risks In August 2018, the Company completed a follow-on public offering of 3,105,000 shares of its common stock (inclusive of 405,000 shares pursuant to the full exercise by the underwriters of their option to purchase additional shares) at a price of $65.00 per share. The aggregate net proceeds received by the Company from the offering, inclusive of the underwriters’ option exercise, were $189.1 million, net of underwriting discounts and commissions and offering expenses payable by the Company. As of September 30, 2018, the Company had generated an accumulated deficit of $87.3 million since inception. As the Company has incurred cumulative losses since inception, transition to recurring profitability is dependent upon the successful development, approval and commercialization of its product candidates and achieving a level of revenues adequate to support the Company’s cost structure. The Company may never achieve recurring profitability, and unless and until it does, the Company will continue to need to raise additional capital. As of September 30, 2018, the Company had cash, cash equivalents and marketable securities of $474.9 million, which management believes is sufficient to fund operations for at least the next 12 months from the date these consolidated financial statements were issued. The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, development by the Company or its competitors of technological innovations, risks of failure of clinical trials, dependence on key personnel, protection of proprietary technology, compliance with government regulations and ability to transition from clinical manufacturing to the commercial production of products.

Income Taxes	9 Months Ended
Income Taxes	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	9. Income Taxes The TCJA was signed into law in December 2017, and has resulted in significant changes to the U.S. corporate income tax system. In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118 (SAB 118), which allows the Company to record provisional amounts for the effects of the TCJA in the period it was enacted for a measurement period not extend beyond one year from the enactment date. In accordance with SAB 118, the Company has determined that the impact of the TCJA to its deferred tax assets and liabilities and valuation allowance as of September 30, 2018 and December 31, 2017 is an estimate and provisional amount. The final impact of the TCJA may differ from this provisional amount due to changes in the Company’s estimates and the issuance of additional regulatory or other guidance. The Company expects to complete its assessment of the final impact of the TCJA within the required measurement period under SAB 118. The Company’s effective tax rate for the three and nine months ended September 30, 2018 differed from the U.S. federal statutory rate of 21%, primarily due to tax credits generated, tax windfall benefits from share-based payments and the expected utilization of U.S. federal net operating loss (NOL) carryforwards. These benefits were partially offset by state taxes and non-deductible expenses. The Company’s net deferred tax assets decreased during the nine months ended September 30, 2018, primarily as a result of the expected utilization of U.S. federal and state NOL carryforwards. The decrease in deferred tax assets was offset by a corresponding decrease in the Company’s valuation allowance resulting in no impact on the Company’s tax provision for the period. The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets. Based on the Company’s history of operating losses, the Company has concluded that it is more likely than not that the benefit of its deferred tax assets will not be realized. Accordingly, the Company has provided a full valuation allowance for its net deferred tax assets as of September 30, 2018 and December 31, 2017.

Related Party Transactions	9 Months Ended
Related Party Transactions	Sep. 30, 2018
Related Party Transactions [Abstract]
Related Party Transactions	10. Related Party Transactions FOXKISER LLP Effective January 2017, the Company entered into a Professional Services Agreement with FOXKISER LLP (FOXKISER), an affiliate of certain stockholders of the Company and an affiliate of a member of the Company’s Board of Directors, pursuant to which the Company pays a fixed monthly fee in consideration for certain strategic planning, development and regulatory services provided by FOXKISER. The agreement expired in December 2017, and effective January 2018 the Company entered into a new Professional Services Agreement with FOXKISER, which has a term of one year and is terminable by either party, at any time, upon 60 days’ prior written notice to the other party. Costs incurred under the agreements with FOXKISER for the three and nine months ended September 30, 2018 were $0.5 million and $1.6 million, respectively. Costs incurred under the agreements with FOXKISER for the three and nine months ended September 30, 2017 were $0.4 million and $1.1 million, respectively. Costs incurred under the agreements with FOXKISER are recorded as research and development expenses in the consolidated statements of operations and comprehensive income (loss). Scientific Founder and Special Advisor In September 2014, the Company entered into an advisory agreement, as amended, with James M. Wilson, M.D., Ph.D., who was formerly the Company’s Chief Scientific Advisor, and is currently the Chairman of the Company’s Scientific Advisory Board and Special Advisor. The agreement required a fixed monthly payment in consideration for scientific advisory services and expired in March 2017, after which the Company no longer deemed Dr. Wilson to be a related party. During the three months ended March 31, 2017, the Company incurred advisory fees of $0.1 million under the agreement, which are recorded as research and development expenses in the consolidated statements of operations and comprehensive income (loss). Pursuant to a new advisory agreement entered into in March 2017 and which expires on December 31, 2018, Dr. Wilson may provide services at no cost to the Company.

Nature of Business - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	9 Months Ended
	Aug. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Class of Stock [Line Items]
Net proceeds from issuance of common stock		$ 189,716	$ 81,994
Accumulated deficit		(87,322)		$ (187,756)
Cash, cash equivalents and marketable securities		$ 474,900
Follow-on Public Offering [Member]
Class of Stock [Line Items]
Number of shares issued during period	3,105,000
Stock issuance price per share	$ 65.00
Net proceeds from issuance of common stock	$ 189,100
Overallotment Option [Member] \| Common Stock [Member]
Class of Stock [Line Items]
Number of shares issued during period	405,000

Summary of Significant Accounting Policies - Schedule of Impact on Consolidated Balance Sheet on Adoption of Topic 606 (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Accounts receivable, current	$ 2,626		$ 473
Accounts receivable, non-current	4,600
Other current assets	4,834		1,412
Prepaid expenses	6,308		5,334
Liabilities:
Accrued expenses and other current liabilities	13,306		9,605
Deferred revenue	600
Other liabilities	691
Stockholders’ equity
Accumulated deficit	(87,322)		$ (187,756)
ASU 2014-09 [Member]
Assets
Accounts receivable, current		$ 1,000
Accounts receivable, non-current		4,850
Other current assets		1,762
Liabilities:
Accrued expenses and other current liabilities		9,710
Other liabilities		819
Stockholders’ equity
Accumulated deficit		(182,953)
Cumulative Adjustment for Adoption of Topic 606 [Member] \| ASU 2014-09 [Member]
Assets
Accounts receivable, current	626	527
Accounts receivable, non-current	4,600	4,850
Other current assets	2,000	350
Prepaid expenses	60
Liabilities:
Accrued expenses and other current liabilities	322	105
Deferred revenue	600
Other liabilities	691	819
Stockholders’ equity
Accumulated deficit	5,673	$ 4,803
Results Without Adoption of Topic 606 \| ASU 2014-09 [Member]
Assets
Accounts receivable, current	2,000
Other current assets	2,834
Prepaid expenses	6,248
Liabilities:
Accrued expenses and other current liabilities	12,984
Stockholders’ equity
Accumulated deficit	$ (92,995)

Summary of Significant Accounting Policies - Schedule of Impact on Consolidated Statement of Cash Flows on Adoption of Topic 606 (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash Flows from Operating Activities:
Net income	$ 95,631	$ (57,172)
Imputed interest income from licensing	(8,362)
Changes in accounts receivable	6,986	150
Changes in prepaid expenses	(974)	(1,450)
Changes in other current assets	(3,072)	(380)
Changes in accrued expenses and other current liabilities	3,438	$ 3,449
Changes in deferred revenue	600
Changes in other liabilities	(128)
Cumulative Adjustment for Adoption of Topic 606 [Member] \| ASU 2014-09 [Member]
Cash Flows from Operating Activities:
Net income	870
Imputed interest income from licensing	(8,362)
Changes in accounts receivable	8,513
Changes in prepaid expenses	(60)
Changes in other current assets	(1,650)
Changes in accrued expenses and other current liabilities	217
Changes in deferred revenue	600
Changes in other liabilities	(128)
Results Without Adoption of Topic 606 \| ASU 2014-09 [Member]
Cash Flows from Operating Activities:
Net income	94,761
Changes in accounts receivable	(1,527)
Changes in prepaid expenses	(914)
Changes in other current assets	(1,422)
Changes in accrued expenses and other current liabilities	$ 3,221

Marketable Securities - Summary of Company Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 332,901	$ 130,018
Unrealized Gains	5	2
Unrealized Losses	(444)	(282)
Fair Value	332,462	129,738
U.S. Government and Federal Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	68,437
Unrealized Losses	(145)
Fair Value	68,292
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	6,806
Fair Value	6,806
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	257,658	130,018
Unrealized Gains	5	2
Unrealized Losses	(299)	(282)
Fair Value	$ 257,364	$ 129,738

Marketable Securities - Additional Information (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
Marketable Securities - Additional Information (Detail)	Sep. 30, 2018 USD ($) Security	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Security	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities remaining maturities greater than three years	$ 0		$ 0		$ 0
Unrealized gains (losses) on available-for-sale securities, before tax	(100,000)	$ 100,000	(200,000)
Unrealized gains (losses) on available-for-sale securities, income tax expense	$ 0	0	$ 0	$ 0
Number of investment grade fixed income security \| Security	106		106
Other-than-temporary impaired			$ 0		$ 0
Maximum [Member]
Schedule Of Available For Sale Securities [Line Items]
Unrealized gains (losses) on available-for-sale securities, before tax				(100,000)
Debt Securities [Member] \| Investment Income [Member] \| Maximum [Member]
Schedule Of Available For Sale Securities [Line Items]
Net realized gains or losses available for sale securities	$ 100,000	$ 100,000	$ 100,000	$ 500,000

Marketable Securities - Summary of Fair Values and Unrealized Losses of Marketable Securities Held by the Company in an Unrealized Loss Position for Less Than 12 months and 12 Months or Greater (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 290,421
Less than 12 Months, Unrealized Losses	(404)	$ (180)
12 Months or Greater, Fair Value	10,588
12 Months or Greater, Unrealized Losses	(40)	(102)
Total, Fair Value	301,009
Total, Unrealized Losses	(444)	(282)
Less than 12 Months, Fair Value		109,238
12 Months or Greater, Fair Value		17,124
Total, Fair Value		126,362
U.S. Government and Federal Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	68,292
Less than 12 Months, Unrealized Losses	(145)
Total, Fair Value	68,292
Total, Unrealized Losses	(145)
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	222,129
Less than 12 Months, Unrealized Losses	(259)	(180)
12 Months or Greater, Fair Value	10,588
12 Months or Greater, Unrealized Losses	(40)	(102)
Total, Fair Value	232,717
Total, Unrealized Losses	$ (299)	(282)
Less than 12 Months, Fair Value		109,238
12 Months or Greater, Fair Value		17,124
Total, Fair Value		$ 126,362

Fair Value of Financial Instruments - Schedule of Fair Value of Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Marketable securities:
Fair Value	$ 332,462	$ 129,738
Cash Equivalents and Marketable Securities [Member]
Marketable securities:
Total cash equivalents and marketable securities	474,867	176,384
Corporate Bonds [Member]
Marketable securities:
Fair Value	257,364	129,738
Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	142,405	46,646
Cash Equivalents [Member] \| Corporate Bonds [Member]
Cash equivalents:
Total cash equivalents	5,644
Certificates of Deposit [Member]
Marketable securities:
Fair Value	6,806
U.S. Government and Federal Agency Securities [Member]
Marketable securities:
Fair Value	68,292
Money Market Mutual Funds [Member] \| Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	136,761	46,646
Significant Other Observable Inputs (Level 2) [Member]
Marketable securities:
Fair Value	332,462	129,738
Significant Other Observable Inputs (Level 2) [Member] \| Cash Equivalents and Marketable Securities [Member]
Marketable securities:
Total cash equivalents and marketable securities	474,867	176,384
Significant Other Observable Inputs (Level 2) [Member] \| Corporate Bonds [Member]
Marketable securities:
Fair Value	257,364	129,738
Significant Other Observable Inputs (Level 2) [Member] \| Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	142,405	46,646
Significant Other Observable Inputs (Level 2) [Member] \| Cash Equivalents [Member] \| Corporate Bonds [Member]
Cash equivalents:
Total cash equivalents	5,644
Significant Other Observable Inputs (Level 2) [Member] \| Certificates of Deposit [Member]
Marketable securities:
Fair Value	6,806
Significant Other Observable Inputs (Level 2) [Member] \| U.S. Government and Federal Agency Securities [Member]
Marketable securities:
Fair Value	68,292
Significant Other Observable Inputs (Level 2) [Member] \| Money Market Mutual Funds [Member] \| Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	$ 136,761	$ 46,646

Property and Equipment Net - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Total property and equipment	$ 25,782	$ 17,254
Accumulated depreciation and amortization	(5,926)	(3,277)
Property and equipment, net	19,856	13,977
Lab Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	11,129	8,561
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,945	1,481
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,872	1,384
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	$ 10,836	$ 5,828

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases (Detail) $ in Thousands	Sep. 30, 2018 USD ($)
Operating Leases Future Minimum Payments Due [Abstract]
2018 (remainder of year)	$ 660
2019	2,946
2020	2,975
2021	2,335
2022	621
Thereafter	479
Total minimum lease payments	$ 10,016

License Revenue - Summary of Changes in Balances of Receivables, Contract Assets and Contract Liabilities (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Receivables and contract assets:
Accounts receivable, current and non-current, Balance at Beginning of Period	$ 5,224	$ 5,850
Accounts receivable, current and non-current, Net Additions (Deductions)	2,002	1,376
Accounts receivable, current and non-current, Balance at End of Period	7,226	7,226
Contract assets, Balance at Beginning of Period		350
Contract assets, Net Additions (Deductions)	2,000	1,650
Contract assets, Balance at End of Period	2,000	2,000
Contract liabilities:
Deferred revenue, Balance at Beginning of Period	600
Deferred revenue, Net Additions (Deductions)		600
Deferred revenue, Balance at End of Period	$ 600	$ 600

License Revenue - Additional Information (Detail1) - January 2018 Amendment Agreement [Member] - USD ($) $ in Millions	1 Months Ended	9 Months Ended
	May 31, 2018	Sep. 30, 2018
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2018-01-01
License Revenue [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, period		1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: (nil)
License Revenue [Line Items]
Sales-based milestone payment exclude transaction price		$ 80.0
Payments due from related party		$ 30.0
Consideration payment due period one		2019-01
Consideration payment due period two		2020-01
Share-based Milestone [Member] \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: (nil)
License Revenue [Line Items]
Increase in transaction price of license	$ 40.0

Stock-based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	9 Months Ended
	Jan. 31, 2018	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense		$ 40,800
Unrecognized stock-based compensation, weighted-average period		2 years 8 months 12 days
Proceeds from exercise of stock options		$ 9,051	$ 752
2015 Equity Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional shares to be issued	1,251,810
Common stock shares authorized for issuance		9,488,413
Shares available for future grants		2,196,366
Weighted-average fair values of options granted		$ 25.57
Exercise of stock options, Shares		1,274,980
Total intrinsic value of options exercised		$ 47,500
2015 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares authorized for issuance		254,000
Shares available for future grants		169,580
Common stock shares issued to participants		36,700

Stock-based Compensation - Summary of Stock Option Activity (Detail) - 2014 and 2015 Equity Incentive Plan [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]
Shares Outstanding, Beginning Balance	5,468
Shares, Granted	1,275
Shares, Exercised	(1,275)
Shares, Cancelled or forfeited	(297)
Shares Outstanding, Ending Balance	5,171	5,468
Shares, Exercisable	2,726
Shares, Vested and expected to vest	5,150
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-average Exercise Price Outstanding, Beginning Balance	$ 10.25
Weighted-average Exercise Price, Granted	38.37
Weighted-average Exercise Price, Exercised	7.22
Weighted-average Exercise Price, Cancelled or forfeited	16.91
Weighted-average Exercise Price, Outstanding, Ending Balance	17.55	$ 10.25
Weighted-average Exercise Price, Exercisable	8.85
Weighted-average Exercise Price, Vested and expected to vest	$ 17.62
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average Remaining Contractual Life (Years) Outstanding	7 years 8 months 12 days	7 years 10 months 24 days
Weighted-average Remaining Contractual Life (Years), Exercisable	6 years 10 months 24 days
Weighted-average Remaining Contractual Life (Years), Vested and expected to vest	7 years 8 months 12 days
Aggregate Intrinsic Value Outstanding	$ 299,646	$ 125,738
Aggregate Intrinsic Value, Exercisable	181,718
Aggregate Intrinsic Value, Vested and expected to vest	$ 298,060

Stock-based Compensation - Summary of Unvested RSUs Activity Under 2015 Plan (Detail) - Restricted Stock Units [Member] shares in Thousands	9 Months Ended
	Sep. 30, 2018 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Unvested, Beginning Balance	40
Shares, Granted	0
Shares, Vested	0
Shares, Forfeited	0
Shares Unvested, Ending Balance	40
Weighted-average Grant Date Fair Value, Unvested Beginning Balance \| $ / shares	$ 20.90
Weighted-average Grant Date Fair Value, Unvested Ending Balance \| $ / shares	$ 20.90

Income Taxes - Additional Information (Detail)	3 Months Ended	9 Months Ended
Income Taxes - Additional Information (Detail)	Sep. 30, 2018	Sep. 30, 2018
Income Tax Disclosure [Abstract]
U.S. federal statutory income tax rate, percent	21.00%	21.00%

Related Party Transactions - Additional Information (Detail) - Research and Development Costs [Member] - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
FOXKISER,LLP [Member] \| Service Agreements [Member]
Related Party Transaction [Line Items]
Related party transaction expense	$ 0.5	$ 0.4		$ 1.6	$ 1.1
Scientific Founder and Special Advisor [Member]
Related Party Transaction [Line Items]
Related party transaction expenses from transactions with related party advisory fees			$ 0.1

Supplemental Disclosures - Schedules of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued personnel costs	$ 6,381	$ 5,789
Accrued external research and development expenses	3,136	2,072
Accrued external general and administrative expenses	1,228	1,078
Accrued income taxes payable	1,140
Accrued licensing costs	646
Accrued purchases of property and equipment	495	430
Other accrued expenses and current liabilities	280	236
Accrued expenses and other current liabilities	$ 13,306	$ 9,605