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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2018	May 04, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
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 Common Stock, Shares Outstanding		31,936,013

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 71,870	$ 46,656
Marketable securities	157,997	114,122
Accounts receivable	25,976	473
Prepaid expenses	4,667	5,334
Other current assets	2,208	1,412
Total current assets	262,718	167,997
Marketable securities	5,917	15,616
Accounts receivable	32,645
Property and equipment, net	14,829	13,977
Restricted cash	225	225
Other assets	883	862
Total assets	317,217	198,677
Current liabilities
Accounts payable	5,007	4,832
Accrued expenses and other current liabilities	9,869	9,605
Total current liabilities	14,876	14,437
Deferred rent, net of current portion	1,225	1,211
Other liabilities	1,776
Total liabilities	17,877	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 March 31, 2018 and December 31, 2017
Common stock; $0.0001 par value; 100,000 shares authorized at March 31, 2018 and December 31, 2017; 31,900 and 31,295 shares issued and outstanding at March 31, 2018 and December 31, 2017, respectively	3	3
Additional paid-in capital	378,954	371,497
Accumulated other comprehensive loss	(903)	(715)
Accumulated deficit	(78,714)	(187,756)
Total stockholders’ equity	299,340	183,029
Total liabilities and stockholders’ equity	$ 317,217	$ 198,677

Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 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	31,900,000	31,295,000
Common stock, shares outstanding	31,900,000	31,295,000

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenues
License revenue	$ 132,391	$ 455
Total revenues	132,391	455
Costs of revenues
Licensing costs	2,408	91
Research and development	19,550	16,619
General and administrative	8,380	6,622
Other operating expenses	28	45
Total operating expenses	30,366	23,377
Income (loss) from operations	102,025	(22,922)
Other Income
Interest income from licensing	1,355
Investment income	859	929
Total other income	2,214	929
Net income (loss)	104,239	(21,993)
Other Comprehensive Loss
Unrealized loss on available-for-sale securities, net of reclassifications and income tax expense	(188)	(539)
Total other comprehensive loss	(188)	(539)
Comprehensive income (loss)	104,051	(22,532)
Net income (loss) applicable to common stockholders	$ 104,239	$ (21,993)
Net income (loss) per share:
Basic	$ 3.30	$ (0.82)
Diluted	$ 3.04	$ (0.82)
Basic	31,632	26,673
Diluted	34,275	26,673

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities
Net income (loss)	$ 104,239	$ (21,993)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities
Stock-based compensation expense	3,291	2,591
Net amortization of premiums and accretion of discounts on marketable debt securities	378	469
Depreciation and amortization	834	554
Net realized gains on sales of marketable securities		(480)
Imputed interest income from licensing	(1,355)
Other non-cash adjustments	10	41
Changes in operating assets and liabilities
Accounts receivable	(51,414)	803
Prepaid expenses	667	(68)
Other current assets	(446)	88
Other assets	(21)	10
Accounts payable	477	2,468
Accrued expenses and other current liabilities	490	(1,920)
Deferred rent	27	(44)
Other liabilities	957
Net cash provided by (used in) operating activities	58,134	(17,481)
Cash flows from investing activities
Purchases of marketable securities	(54,267)	(5,188)
Maturities of marketable securities	19,525	10,500
Sales of marketable securities		780
Purchases of property and equipment	(2,344)	(2,929)
Net cash provided by (used in) investing activities	(37,086)	3,163
Cash flows from financing activities
Proceeds from exercise of stock options	3,824	161
Proceeds from issuance of common stock under employee stock purchase plan	342	147
Proceeds from public offering of common stock, net of underwriting discounts and commissions		71,299
Issuance costs for public offering of common stock		(84)
Net cash provided by financing activities	4,166	71,523
Net increase in cash and cash equivalents and restricted cash	25,214	57,205
Cash and cash equivalents and restricted cash
Beginning of period	46,881	25,065
End of period	$ 72,095	82,270
Supplemental disclosures of non-cash investing and financing activities
Issuance costs for public offering of common stock in accounts payable and accrued expenses		$ 328

Nature of Business	3 Months Ended
Nature of Business	Mar. 31, 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 As of March 31, 2018, the Company had generated an accumulated deficit of $78.7 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 March 31, 2018, the Company had cash, cash equivalents and marketable securities of $235.8 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.

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 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 March 31, 2018, and the results of its operations and its cash flows for the interim periods ended March 31, 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 consists of consideration due to the Company resulting from its license agreements with NAV Technology Licensees. Accounts receivable includes 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 is not contractually payable to the Company is 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 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 March 31, 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, however 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 only identified performance obligations in the Company’s license agreements are for the delivery of intellectual property licenses to licensees. The transaction price for each license agreement is fully allocated to the delivery of the license(s) and is recognized as revenue upon the delivery of the license(s) to the licensee, which generally occurs upon the execution of the license agreement.

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. The Company has determined that none of the options provided to licensees in its license agreements represent material rights to licensees, and therefore do not represent performance obligations of the Company 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 and recognizes changes to the transaction price as revenue during the period, provided that the associated license has been delivered to the licensee. 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, sales-based milestone payments 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 is 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 is 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.

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.

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 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 are not 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
Other current assets	$	1,412		$	350	$	1,762
Accounts receivable, non-current	$	—		$	4,850	$	4,850

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 period ended March 31, 2018 (in thousands):

	As of March 31, 2018
				Impact of		Results Without
				Adoption of		Adoption of
	As Reported			Topic 606		Topic 606
Consolidated Balance Sheet
Assets:
Accounts receivable, current	$	25,976		$	25,903	$	73
Accounts receivable, non-current	$	32,645		$	32,645	$	—
Other current assets	$	2,208		$	250	$	1,958

Liabilities:
Accrued expenses and other current liabilities	$	9,869		$	131	$	9,738
Other liabilities	$	1,776		$	1,776	$	—

Stockholdersʼ Equity:
Accumulated deficit	$	(78,714	)	$	56,891	$	(135,605	)

	Three Months Ended March 31, 2018
			Impact of		Results Without
			Adoption of		Adoption of
	As Reported		Topic 606		Topic 606
Consolidated Statement of Operations
Revenues:
License revenue	$	132,391	$	51,716	$	80,675

Expenses:
Licensing costs	$	2,408	$	983	$	1,425

Other Income:
Interest income from licensing	$	1,355	$	1,355	$	—

Net Income	$	104,239	$	52,088	$	52,151

Net Income Per Share:
Basic	$	3.30	$	1.65	$	1.65
Diluted	$	3.04	$	1.52	$	1.52

	Three Months Ended March 31, 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	$	104,239		$	52,088		$	52,151
Imputed interest income from licensing	$	(1,355	)	$	(1,355	)	$	—
Changes in accounts receivable	$	(51,414	)	$	(51,816	)	$	402
Changes in other current assets	$	(446	)	$	100		$	(546	)
Changes in accrued expenses and other current liabilities	$	490		$	26		$	464
Changes in other liabilities	$	957		$	957		$	—

The most significant effect that the adoption of Topic 606 had on the results of operations for the three months ended March 31, 2018, as compared to what results would have been under Topic 605, is related to the amount of revenue recognized under the Company’s January 2018 amendment to its license agreement with AveXis, Inc. (AveXis) for the development and commercialization of treatments for spinal muscular atrophy (SMA). Under Topic 606, the Company recognized the present value of all fixed consideration due to the Company under the amendment as revenue during the period, including the present value of the two $30.0 million payments due to the Company from AveXis in January 2019 and January 2020. Under Topic 605, the Company would not have recognized such revenue until it became fixed and determinable and collectability was reasonably assured. Please refer to Note 7 for further information on license revenue and the Company’s accounting analysis for the amended license with AveXis.

Other recently adopted accounting pronouncements

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 share-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 includes 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):

	March 31, 2018		March 31, 2017
Cash and cash equivalents	$	71,870	$	82,045
Restricted cash		225		225
Total cash and cash equivalents and restricted cash	$	72,095	$	82,270

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 it 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 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. 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.

Marketable Securities

3 Months Ended

Mar. 31, 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
March 31, 2018
U.S. government and federal agency securities	$	6,969	$	—	$	(5	)	$	6,964
Commercial paper		3,484		—		—		$	3,484
Corporate bonds		153,929		—		(463	)	$	153,466
	$	164,382	$	—	$	(468	)	$	163,914

	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 March 31, 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 March 31, 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 ended March 31, 2018, the Company recognized net unrealized losses on available-for-sale securities of $0.2 million and income tax expense of $0 in other comprehensive loss for the period. The Company did not recognize any realized gains or losses on the sale or maturity of available-for-sale securities during the three months ended March 31, 2018. During the three months ended March 31, 2017, the Company recognized net unrealized losses on available-for-sale securities of $0.1 million and income tax expense of $0 in other comprehensive loss for the period. The Company recognized net realized gains of $0.5 million on the sale or maturity of marketable securities during the three months ended March 31, 2017, 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 and greater 12 months (in thousands):

	Less than 12 Months					12 Months or Greater					Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
March 31, 2018
U.S. government and federal agency securities	$	6,964	$	(5	)	$	—	$	—		$	6,964	$	(5	)
Corporate bonds		120,338		(319	)		28,073		(144	)		148,411		(463	)
	$	127,302	$	(324	)	$	28,073	$	(144	)	$	155,375	$	(468	)

	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 March 31, 2018, securities held by the Company which were in an unrealized loss position consisted of 54 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 March 31, 2018 or December 31, 2017.

Fair Value of Financial Instruments

3 Months Ended

Mar. 31, 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. Cash equivalents consist of money market mutual funds. Marketable securities consist of fixed income debt 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
March 31, 2018
Money market mutual funds (cash equivalents)	$	—	$	71,847	$	—	$	71,847
U.S. government and federal agency securities (marketable securities)		—		6,964		—		6,964
Commercial paper (marketable securities)		—		3,484		—		3,484
Corporate bonds (marketable securities)		—		153,466		—		153,466
	$	—	$	235,761	$	—	$	235,761

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

There were no transfers of financial instruments between levels of the fair value hierarchy during the three months ended March 31, 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 March 31, 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. As of March 31, 2018, accounts receivable from AveXis accounted for 93% of the Company’s total accounts receivable, current and non-current.

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 March 31, 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 March 31, 2018. No impairment losses on non-marketable equity securities were recorded during the three months ended March 31, 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

Property and Equipment, Net

3 Months Ended

Mar. 31, 2018

Property Plant And Equipment [Abstract]

Property and Equipment, Net

5. Property and Equipment, Net

Property and equipment, net consists of the following (in thousands):

	March 31, 2018			December 31, 2017
Computer equipment and software	$	1,638		$	1,481
Lab equipment		8,945			8,561
Furniture and fixtures		1,495			1,384
Leasehold improvements		6,841			5,828
Total property and equipment		18,919			17,254
Accumulated depreciation and amortization		(4,090	)		(3,277	)
Property and equipment, net	$	14,829		$	13,977

Commitments and Contingencies

3 Months Ended

Mar. 31, 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 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 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 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 March 31, 2018, the Company has recorded restricted cash of $0.2 million as collateral to the financial institution which issued the letter of credit.

As of March 31, 2018, future minimum lease payments under non-cancelable operating leases are as follows (in thousands):

	Operating
	Leases
2018 (remainder of year)	$	1,714
2019		2,296
2020		2,309
2021		1,719
2022		621
Thereafter		479
Total minimum lease payments	$	9,138

Licenses Granted to the Company

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 costs of reagent sales 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 March 31,
	2018		2017
Sublicense fees	$	—	$	46
Maintenance of licensed patents		18		84
	$	18	$	130

As of March 31, 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 March 31, 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 March 31,
	2018		2017
Sublicense fees	$	2,033	$	46
Maintenance of licensed patents		393		145
	$	2,426	$	191

As of March 31, 2018 and December 31, 2017, the Company had accrued $3.8 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.

Regents of the University of Minnesota. 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 certain patent rights 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, for a total of less than $0.1 million. Under the terms of the agreement, the Company is obligated to pay Minnesota annual maintenance fees of less than $0.1 million per year on each anniversary date of the agreement. Additionally, the Company is obligated to pay royalties on net sales and sublicense fees, if any, and up to $0.1 million per licensed product 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 of less than $0.1 million. From the inception of the agreement through March 31, 2018, the Company has incurred less than $0.1 million for milestones that have been achieved or are deemed probable of achievement.

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

	Three Months Ended March 31,
	2018		2017
Sublicense fees	$	375	$	—
Maintenance of licensed patents		12		6
	$	387	$	6

As of March 31, 2018 and December 31, 2017, the Company had accrued $0.4 million and $0.1 million, respectively, in expenses payable to Minnesota under the license agreement, which are included in accounts payable and accrued expenses on the Company’s consolidated balance sheets.

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 March 31, 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. This matter is in its early stages and the Company is unable to estimate the outcome of this matter but intends to assist Penn in vigorously defending this patent. The EPO has scheduled oral proceedings to begin on October 26, 2018. As of March 31, 2018, the Company has not recorded any liabilities related to this matter.

License Revenue	3 Months Ended
License Revenue	Mar. 31, 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 are not adjusted and continue to be reported in accordance with Topic 605. 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 sales-based milestone payments, 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 March 31, 2018, the Company’s license agreements, excluding additional licenses that could be granted upon the exercise of options by licensees, could result in aggregate milestone fees payable to the Company of up to $26.1 million upon the commencement of various stages of clinical trials, $46.0 million upon the submission of regulatory approval filings, $105.5 million upon the approval of commercial products by regulatory agencies and $212.0 million upon the achievement of specified sales targets for licensed products. 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. During the three months ended March 31, 2018, the Company recognized $0.3 million of license revenue for licenses delivered to licensees in prior periods as a result of increases in the transaction prices of its license agreements for development milestones that were previously not considered probable of achievement. Additionally, the Company recognized $0.2 million of interest income from licensing during the three months ended March 31, 2018 from licenses delivered in prior periods which contained significant financing components. As of March 31, 2018, the Company had recorded current accounts receivable of $26.0 million and non-current accounts receivable of $32.6 million under its license agreements which represent fixed license fees and annual maintenance fees payable to the Company over the term of the agreements, net of discounts for significant financing components. As of March 31, 2018, the Company had recorded contract assets of $0.3 million which represent development milestones that are considered probable of achievement but have not yet been achieved and are included in other current assets on the consolidated balance sheet. As of March 31, 2018, the Company had not recorded any contract liabilities related to contracts with customers and has no material undelivered performance obligations. As of March 31, 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. 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 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 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 is 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 milestone payments described in (iii) above, has not yet been paid to the Company, AveXis will be obliged 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 April 2018, AveXis announced that it had entered into an agreement and plan of merger pursuant to which it will be acquired by Novartis AG. AveXis announced that it expects to complete the transaction in mid-2018, pending the successful completion of a tender offer and the satisfaction of all other closing conditions. The Company expects that the transaction, if completed, will qualify as a change of control of AveXis under the January 2018 Amendment. Accounting Analysis The January 2018 Amendment has been 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 March 31, 2018, the transaction price of the original March 2014 License was $3.5 million. The transaction price of $3.5 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. 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 and as of March 31, 2018, 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. The transaction price of $132.1 million includes the following fixed consideration: (i) the $80.0 million payment in January 2018, (ii) the present value of the $30.0 million payment due in January 2019 and (iii) the present value of the $30.0 million payment due in January 2020. The discounted portion of the license fees represents the financing benefit provided to AveXis and is recognized as interest income from licensing over the financing term of two years from the execution of the amendment. Variable consideration under the January 2018 Amendment which has been excluded from the transaction price includes sales-based milestone payments of $120.0 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. The acceleration of payments under the January 2018 Amendment which would occur upon a change of control of AveXis has not been considered in determining the transaction price as of March 31, 2018 and will not be accounted for until such event occurs and has been completed in full. During the three months ended March 31, 2018, the Company recognized license revenue of $132.1 million and interest income from licensing of $1.2 million 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 March 31, 2018, the Company had recorded $53.5 million of accounts receivable from AveXis under the March 2014 License, as amended, of which $25.4 million are included in current assets and $28.1 million are included in non-current assets on the consolidated balance sheets. During the three months ended March 31, 2017, the Company recognized license revenue of $0.1 million 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 the March 2014 License under the requirements of Topic 605.

Stock-based Compensation

3 Months Ended

Mar. 31, 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 March 31, 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 2,429,185 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 March 31,
	2018		2017
Stock options	$	3,122	$	2,439
Restricted stock units		68		69
Employee stock purchase plan		101		83
	$	3,291	$	2,591

As of March 31, 2018, the Company had $37.6 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 3.0 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 March 31,
	2018		2017
Research and development	$	1,530	$	1,238
General and administrative		1,761		1,353
	$	3,291	$	2,591

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		914		$	32.07
Exercised		(587	)	$	6.57
Cancelled or forfeited		(169	)	$	15.35
Outstanding at March 31, 2018		5,626		$	14.03		8.0	$	91,837
Exercisable at March 31, 2018		2,825		$	7.42		7.3	$	63,385
Vested and expected to vest at March 31, 2018		5,626		$	14.03		8.0	$	91,837

(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 three months ended March 31, 2018 was $21.42. During the three months ended March 31, 2018, the total number of stock options exercised was 586,694, resulting in total proceeds of $3.8 million. The total intrinsic value of options exercised during the three months ended March 31, 2018 was $13.7 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 March 31, 2018		40	$	20.90

Employee Stock Purchase Plan

As of March 31, 2018, the total number of shares of common stock authorized for issuance under the 2015 ESPP was 254,000, of which 186,752 remain available for future issuance. During the three months ended March 31, 2018, 19,528 shares of common stock were issued under the 2015 ESPP.

Income Taxes	3 Months Ended
Income Taxes	Mar. 31, 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 March 31, 2018 and December 31, 2017 is a reasonable 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 net deferred tax assets decreased during the three months ended March 31, 2018, primarily as a result of the expected utilization of federal and state net operating loss 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 March 31, 2018 and December 31, 2017.

Related Party Transactions	3 Months Ended
Related Party Transactions	Mar. 31, 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 months ended March 31, 2018 and 2017 were $0.5 million and $0.4 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., the Company’s Scientific Founder and Special Advisor (formerly the Company’s Chief Scientific Advisor), who is also the Chairman of the Company’s Scientific Advisory Board. The agreement required a fixed monthly payment in consideration for scientific advisory services and expired in March 2017. Dr. Wilson will continue to provide services at no cost to the Company pursuant to a new advisory agreement entered into in March 2017. During the three months ended March 31, 2017, the Company incurred advisory fees of $0.1 million under the agreements. Additionally, in 2014 and 2015 the Company granted options to purchase a total of 211,600 shares of common stock to Dr. Wilson as compensation for the advisory services to be provided to the Company, which vest partially upon the completion of service conditions and partially upon the achievement of specified performance conditions as set forth in the award agreements.

Net Income (Loss) Per Share

3 Months Ended

Mar. 31, 2018

Earnings Per Share [Abstract]

Net Income (Loss) Per Share

11. Net Income (Loss) Per Share

The computations of basic and diluted net income (loss) per share are as follows (in thousands, except per share data):

	Three Months Ended March 31,
	2018		2017
Basic net income (loss) per share:
Net income (loss) applicable to common stockholders	$	104,239	$	(21,993	)
Shares used in computation:
Weighted-average common shares outstanding		31,632		26,673
Basic net income (loss) per share	$	3.30	$	(0.82	)
Diluted net income (loss) per share:
Net income (loss) applicable to common stockholders	$	104,239	$	(21,993	)
Shares used in computation:
Weighted-average common shares outstanding		31,632		26,673
Stock options		2,620		—
Restricted stock units		22		—
Employee stock purchase plan		1		—
Weighted-average diluted common shares		34,275		26,673
Diluted net income (loss) per share	$	3.04	$	(0.82	)

For periods in which the Company incurred net losses applicable to common stockholders, common stock equivalents are excluded from the calculation of diluted net loss per share as their effect would be anti-dilutive, and accordingly, basic and diluted net loss per share are the same for such periods. Outstanding stock options with exercise prices greater than the average market price of common stock are excluded from the calculation of diluted net income (loss) per share as their effect would be anti-dilutive. The following potentially dilutive common stock equivalents outstanding at the end of the period were excluded from the computations of weighted-average diluted common shares for the periods indicated as their effects would be anti-dilutive (in thousands):

	Three Months Ended March 31,
	2018		2017
Stock options issued and outstanding		1,582		5,724
Unvested restricted stock units outstanding		—		40
Employee stock purchase plan		—		16
		1,582		5,780

Supplemental Disclosures

3 Months Ended

Mar. 31, 2018

Payables And Accruals [Abstract]

Supplemental Disclosures

12. Supplemental Disclosures

Accrued expenses and other current liabilities consists of the following (in thousands):

	March 31, 2018		December 31, 2017
Accrued personnel costs	$	3,771	$	5,789
Accrued external research and development expenses		2,851		2,072
Accrued licensing costs		1,555		—
Accrued external general and administrative expenses		1,184		1,078
Accrued purchases of property and equipment		86		430
Other accrued expenses and current liabilities		422		236
	$	9,869	$	9,605

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 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

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
Other current assets	$	1,412		$	350	$	1,762
Accounts receivable, non-current	$	—		$	4,850	$	4,850

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 period ended March 31, 2018 (in thousands):

	As of March 31, 2018
				Impact of		Results Without
				Adoption of		Adoption of
	As Reported			Topic 606		Topic 606
Consolidated Balance Sheet
Assets:
Accounts receivable, current	$	25,976		$	25,903	$	73
Accounts receivable, non-current	$	32,645		$	32,645	$	—
Other current assets	$	2,208		$	250	$	1,958

Liabilities:
Accrued expenses and other current liabilities	$	9,869		$	131	$	9,738
Other liabilities	$	1,776		$	1,776	$	—

Stockholdersʼ Equity:
Accumulated deficit	$	(78,714	)	$	56,891	$	(135,605	)

	Three Months Ended March 31, 2018
			Impact of		Results Without
			Adoption of		Adoption of
	As Reported		Topic 606		Topic 606
Consolidated Statement of Operations
Revenues:
License revenue	$	132,391	$	51,716	$	80,675

Expenses:
Licensing costs	$	2,408	$	983	$	1,425

Other Income:
Interest income from licensing	$	1,355	$	1,355	$	—

Net Income	$	104,239	$	52,088	$	52,151

Net Income Per Share:
Basic	$	3.30	$	1.65	$	1.65
Diluted	$	3.04	$	1.52	$	1.52

	Three Months Ended March 31, 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	$	104,239		$	52,088		$	52,151
Imputed interest income from licensing	$	(1,355	)	$	(1,355	)	$	—
Changes in accounts receivable	$	(51,414	)	$	(51,816	)	$	402
Changes in other current assets	$	(446	)	$	100		$	(546	)
Changes in accrued expenses and other current liabilities	$	490		$	26		$	464
Changes in other liabilities	$	957		$	957		$	—

Other recently adopted accounting pronouncements

	March 31, 2018		March 31, 2017
Cash and cash equivalents	$	71,870	$	82,045
Restricted cash		225		225
Total cash and cash equivalents and restricted cash	$	72,095	$	82,270

Recent accounting pronouncements not yet adopted

Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2018

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
Other current assets	$	1,412		$	350	$	1,762
Accounts receivable, non-current	$	—		$	4,850	$	4,850

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 period ended March 31, 2018 (in thousands):

	As of March 31, 2018
				Impact of		Results Without
				Adoption of		Adoption of
	As Reported			Topic 606		Topic 606
Consolidated Balance Sheet
Assets:
Accounts receivable, current	$	25,976		$	25,903	$	73
Accounts receivable, non-current	$	32,645		$	32,645	$	—
Other current assets	$	2,208		$	250	$	1,958

Liabilities:
Accrued expenses and other current liabilities	$	9,869		$	131	$	9,738
Other liabilities	$	1,776		$	1,776	$	—

Stockholdersʼ Equity:
Accumulated deficit	$	(78,714	)	$	56,891	$	(135,605	)

	Three Months Ended March 31, 2018
			Impact of		Results Without
			Adoption of		Adoption of
	As Reported		Topic 606		Topic 606
Consolidated Statement of Operations
Revenues:
License revenue	$	132,391	$	51,716	$	80,675

Expenses:
Licensing costs	$	2,408	$	983	$	1,425

Other Income:
Interest income from licensing	$	1,355	$	1,355	$	—

Net Income	$	104,239	$	52,088	$	52,151

Net Income Per Share:
Basic	$	3.30	$	1.65	$	1.65
Diluted	$	3.04	$	1.52	$	1.52

	Three Months Ended March 31, 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	$	104,239		$	52,088		$	52,151
Imputed interest income from licensing	$	(1,355	)	$	(1,355	)	$	—
Changes in accounts receivable	$	(51,414	)	$	(51,816	)	$	402
Changes in other current assets	$	(446	)	$	100		$	(546	)
Changes in accrued expenses and other current liabilities	$	490		$	26		$	464
Changes in other liabilities	$	957		$	957		$	—

Marketable Securities (Tables)

3 Months Ended

Mar. 31, 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
March 31, 2018
U.S. government and federal agency securities	$	6,969	$	—	$	(5	)	$	6,964
Commercial paper		3,484		—		—		$	3,484
Corporate bonds		153,929		—		(463	)	$	153,466
	$	164,382	$	—	$	(468	)	$	163,914

	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 and Greater 12 Months

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 and greater 12 months (in thousands):

	Less than 12 Months					12 Months or Greater					Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
March 31, 2018
U.S. government and federal agency securities	$	6,964	$	(5	)	$	—	$	—		$	6,964	$	(5	)
Corporate bonds		120,338		(319	)		28,073		(144	)		148,411		(463	)
	$	127,302	$	(324	)	$	28,073	$	(144	)	$	155,375	$	(468	)

	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	)

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

Fair Value of Financial Instruments (Tables)

3 Months Ended

Mar. 31, 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
March 31, 2018
Money market mutual funds (cash equivalents)	$	—	$	71,847	$	—	$	71,847
U.S. government and federal agency securities (marketable securities)		—		6,964		—		6,964
Commercial paper (marketable securities)		—		3,484		—		3,484
Corporate bonds (marketable securities)		—		153,466		—		153,466
	$	—	$	235,761	$	—	$	235,761

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

Property and Equipment, Net (Tables)

3 Months Ended

Mar. 31, 2018

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment

Property and equipment, net consists of the following (in thousands):

	March 31, 2018			December 31, 2017
Computer equipment and software	$	1,638		$	1,481
Lab equipment		8,945			8,561
Furniture and fixtures		1,495			1,384
Leasehold improvements		6,841			5,828
Total property and equipment		18,919			17,254
Accumulated depreciation and amortization		(4,090	)		(3,277	)
Property and equipment, net	$	14,829		$	13,977

Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2018

Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases

As of March 31, 2018, future minimum lease payments under non-cancelable operating leases are as follows (in thousands):

	Operating
	Leases
2018 (remainder of year)	$	1,714
2019		2,296
2020		2,309
2021		1,719
2022		621
Thereafter		479
Total minimum lease payments	$	9,138

The Trustees of the University of Pennsylvania [Member]

Summary of Expenses Incurred by Company

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

	Three Months Ended March 31,
	2018		2017
Sublicense fees	$	—	$	46
Maintenance of licensed patents		18		84
	$	18	$	130

GlaxoSmithKline LLC [Member]

Summary of Expenses Incurred by Company

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

	Three Months Ended March 31,
	2018		2017
Sublicense fees	$	2,033	$	46
Maintenance of licensed patents		393		145
	$	2,426	$	191

Regents of the University of Minnesota [Member]

Summary of Expenses Incurred by Company

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

	Three Months Ended March 31,
	2018		2017
Sublicense fees	$	375	$	—
Maintenance of licensed patents		12		6
	$	387	$	6

Stock-based Compensation (Tables)

3 Months Ended

Mar. 31, 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 March 31,
	2018		2017
Stock options	$	3,122	$	2,439
Restricted stock units		68		69
Employee stock purchase plan		101		83
	$	3,291	$	2,591

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 March 31,
	2018		2017
Research and development	$	1,530	$	1,238
General and administrative		1,761		1,353
	$	3,291	$	2,591

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 March 31, 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		914		$	32.07
Exercised		(587	)	$	6.57
Cancelled or forfeited		(169	)	$	15.35
Outstanding at March 31, 2018		5,626		$	14.03		8.0	$	91,837
Exercisable at March 31, 2018		2,825		$	7.42		7.3	$	63,385
Vested and expected to vest at March 31, 2018		5,626		$	14.03		8.0	$	91,837

(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.

Net Income (Loss) Per Share (Tables)

3 Months Ended

Mar. 31, 2018

Earnings Per Share [Abstract]

Computations of Basic and Diluted Net Income (Loss) Per Share

The computations of basic and diluted net income (loss) per share are as follows (in thousands, except per share data):

	Three Months Ended March 31,
	2018		2017
Basic net income (loss) per share:
Net income (loss) applicable to common stockholders	$	104,239	$	(21,993	)
Shares used in computation:
Weighted-average common shares outstanding		31,632		26,673
Basic net income (loss) per share	$	3.30	$	(0.82	)
Diluted net income (loss) per share:
Net income (loss) applicable to common stockholders	$	104,239	$	(21,993	)
Shares used in computation:
Weighted-average common shares outstanding		31,632		26,673
Stock options		2,620		—
Restricted stock units		22		—
Employee stock purchase plan		1		—
Weighted-average diluted common shares		34,275		26,673
Diluted net income (loss) per share	$	3.04	$	(0.82	)

Schedules for Computation of Diluted Weighted-Average Shares Outstanding

The following potentially dilutive common stock equivalents outstanding at the end of the period were excluded from the computations of weighted-average diluted common shares for the periods indicated as their effects would be anti-dilutive (in thousands):

	Three Months Ended March 31,
	2018		2017
Stock options issued and outstanding		1,582		5,724
Unvested restricted stock units outstanding		—		40
Employee stock purchase plan		—		16
		1,582		5,780

Supplemental Disclosures (Tables)

3 Months Ended

Mar. 31, 2018

Payables And Accruals [Abstract]

Schedules of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consists of the following (in thousands):

	March 31, 2018		December 31, 2017
Accrued personnel costs	$	3,771	$	5,789
Accrued external research and development expenses		2,851		2,072
Accrued licensing costs		1,555		—
Accrued external general and administrative expenses		1,184		1,078
Accrued purchases of property and equipment		86		430
Other accrued expenses and current liabilities		422		236
	$	9,869	$	9,605

Nature of Business - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Variable Interests And Equity Method Investments Disclosure [Abstract]
Accumulated deficit	$ (78,714)	$ (187,756)
Cash, cash equivalents and marketable securities	$ 235,800

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Percentage of ownership interest	20.00%
Accumulated deficit	$ (78,714)		$ (187,756)
AveXis, Inc. [Member]
Significant Accounting Policies [Line Items]
Payments due from related party	$ 30,000
Consideration payment due period one	2019-01
Consideration payment due period two	2020-01
ASU 2014-09 [Member]
Significant Accounting Policies [Line Items]
Accumulated deficit		$ (182,953)
Reduction in Accumulated Deficit [Member] \| ASU 2014-09 [Member]
Significant Accounting Policies [Line Items]
Accumulated deficit	$ 56,891	$ 4,803

Summary of Significant Accounting Policies - Schedule of Impact on Consolidated Balance Sheet on Adoption of Topic 606 (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Accounts receivable, current	$ 25,976		$ 473
Other current assets	2,208		1,412
Accounts receivable, non-current	32,645
Liabilities:
Accrued expenses and other current liabilities	9,869		9,605
Other liabilities	1,776
Stockholders’ equity
Accumulated deficit	(78,714)		$ (187,756)
ASU 2014-09 [Member]
Assets
Accounts receivable, current		$ 1,000
Other current assets		1,762
Accounts receivable, non-current		4,850
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	25,903	527
Other current assets	250	350
Accounts receivable, non-current	32,645	4,850
Liabilities:
Accrued expenses and other current liabilities	131	105
Other liabilities	1,776	819
Stockholders’ equity
Accumulated deficit	56,891	$ 4,803
Results Without Adoption of Topic 606 \| ASU 2014-09 [Member]
Assets
Accounts receivable, current	73
Other current assets	1,958
Liabilities:
Accrued expenses and other current liabilities	9,738
Stockholders’ equity
Accumulated deficit	$ (135,605)

Summary of Significant Accounting Policies - Schedule of Impact on Consolidated Statement of Operations on Adoption of Topic 606 (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenues
License revenue	$ 132,391	$ 455
Expenses
Licensing costs	2,408	91
Other Income
Interest income from licensing	1,355
Net Income	$ 104,239	$ (21,993)
Net income (loss) per share:
Basic	$ 3.30	$ (0.82)
Diluted	$ 3.04	$ (0.82)
Cumulative Adjustment for Adoption of Topic 606 [Member] \| ASU 2014-09 [Member]
Revenues
License revenue	$ 51,716
Expenses
Licensing costs	983
Other Income
Interest income from licensing	1,355
Net Income	$ 52,088
Net income (loss) per share:
Basic	$ 1.65
Diluted	$ 1.52
Results Without Adoption of Topic 606 \| ASU 2014-09 [Member]
Revenues
License revenue	$ 80,675
Expenses
Licensing costs	1,425
Other Income
Net Income	$ 52,151
Net income (loss) per share:
Basic	$ 1.65
Diluted	$ 1.52

Summary of Significant Accounting Policies - Schedule of Impact on Consolidated Statement of Cash Flows on Adoption of Topic 606 (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities
Net Income	$ 104,239	$ (21,993)
Imputed interest income from licensing	(1,355)
Changes in accounts receivable	(51,414)	803
Changes in other current assets	(446)	88
Changes in accrued expenses and other current liabilities	490	$ (1,920)
Changes in other liabilities	957
Cumulative Adjustment for Adoption of Topic 606 [Member] \| ASU 2014-09 [Member]
Cash flows from operating activities
Net Income	52,088
Imputed interest income from licensing	(1,355)
Changes in accounts receivable	(51,816)
Changes in other current assets	100
Changes in accrued expenses and other current liabilities	26
Changes in other liabilities	957
Results Without Adoption of Topic 606 \| ASU 2014-09 [Member]
Cash flows from operating activities
Net Income	52,151
Changes in accounts receivable	402
Changes in other current assets	(546)
Changes in accrued expenses and other current liabilities	$ 464

Marketable Securities - Summary of Company Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 164,382	$ 130,018
Unrealized Gains		2
Unrealized Losses	(468)	(282)
Fair Value	163,914	129,738
U.S. Government and Federal Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	6,969
Unrealized Losses	(5)
Fair Value	6,964
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	153,929	130,018
Unrealized Gains		2
Unrealized Losses	(463)	(282)
Fair Value	153,466	$ 129,738
Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	3,484
Fair Value	$ 3,484

Marketable Securities - Additional Information (Detail)	3 Months Ended		12 Months Ended
Marketable Securities - Additional Information (Detail)	Mar. 31, 2018 USD ($) Security	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities remaining maturities greater than three years	$ 0		$ 0
Unrealized losses on available-for-sale securities, before tax	200,000	$ 100,000
Unrealized losses on available-for-sale securities, income tax expense	$ 0	0
Net realized gains or losses available for sale securities		480,000
Number of investment grade fixed income security \| Security	54
Other-than-temporary impaired	$ 0		$ 0
Common Equity Securities [Member] \| Investment Income
Schedule Of Available For Sale Securities [Line Items]
Net realized gains or losses available for sale securities	$ 0	$ 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 and Greater 12 Months (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 127,302	$ 109,238
Less than 12 Months, Unrealized Losses	(324)	(180)
12 Months or Greater, Fair Value	28,073	17,124
12 Months or Greater, Unrealized Losses	(144)	(102)
Total, Fair Value	155,375	126,362
Total, Unrealized Losses	(468)	(282)
U.S. Government and Federal Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	6,964
Less than 12 Months, Unrealized Losses	(5)
Total, Fair Value	6,964
Total, Unrealized Losses	(5)
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	120,338	109,238
Less than 12 Months, Unrealized Losses	(319)	(180)
12 Months or Greater, Fair Value	28,073	17,124
12 Months or Greater, Unrealized Losses	(144)	(102)
Total, Fair Value	148,411	126,362
Total, Unrealized Losses	$ (463)	$ (282)

Fair Value of Financial Instruments - Schedule of Fair Value of Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	$ 163,914	$ 129,738
Assets fair value disclosure	235,761	176,384
Corporate Bonds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	153,466	129,738
Commercial Paper [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	3,484
U.S. Government and Federal Agency Securities [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	6,964
Money Market Mutual Funds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Cash equivalents	71,847	46,646
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets fair value disclosure	235,761	176,384
Significant Other Observable Inputs (Level 2) [Member] \| Corporate Bonds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	153,466	129,738
Significant Other Observable Inputs (Level 2) [Member] \| Commercial Paper [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	3,484
Significant Other Observable Inputs (Level 2) [Member] \| U.S. Government and Federal Agency Securities [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	6,964
Significant Other Observable Inputs (Level 2) [Member] \| Money Market Mutual Funds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Cash equivalents	$ 71,847	$ 46,646

Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value hierarchy level 1 to level 2 transfers amount	$ 0
Fair value hierarchy level 2 to level 1 transfers amount	0
Non-marketable equity securities	400,000		$ 400,000
impairment losses on non-marketable equity securities	$ 0	$ 0
AveXis, Inc. [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Percentage of Accounts receivable	93.00%

Property and Equipment Net - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Total property and equipment	$ 18,919	$ 17,254
Accumulated depreciation and amortization	(4,090)	(3,277)
Property and equipment, net	14,829	13,977
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,638	1,481
Lab Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	8,945	8,561
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,495	1,384
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	$ 6,841	$ 5,828

Commitments and Contingencies - Additional Information (Detail) - USD ($)	1 Months Ended							3 Months Ended		12 Months Ended	41 Months Ended	109 Months Ended
	Nov. 30, 2016	May 31, 2016	Jan. 31, 2016	Sep. 30, 2015	Mar. 31, 2015	Nov. 30, 2014	Mar. 31, 2009	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Mar. 31, 2018	Mar. 31, 2018
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
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								3,800,000		300,000	3,800,000	3,800,000
Milestone payment obligation							$ 1,500,000
Related party transaction expense								2,426,000	$ 191,000
GlaxoSmithKline LLC [Member] \| Milestone Fees [Member]
Other Commitments [Line Items]
Related party transaction expense												500,000
Regents of the University of Minnesota [Member]
Other Commitments [Line Items]
Related party transaction expense								387,000	$ 6,000
Regents of the University of Minnesota [Member] \| Maximum [Member]
Other Commitments [Line Items]
Up-front fee paid and patent maintenance expenses reimbursed						$ 100,000
Up-front fee paid	$ 100,000
Amounts due under agreement								$ 400,000		$ 100,000	400,000	$ 400,000
Regents of the University of Minnesota [Member] \| Maximum [Member] \| Milestone Fees [Member]
Other Commitments [Line Items]
Related party transaction expense											$ 100,000
Regents of the University of Minnesota [Member] \| Maximum [Member] \| Annual License Maintenance Fee [Member]
Other Commitments [Line Items]
Contractual obligation						100,000
Regents of the University of Minnesota [Member] \| Maximum [Member] \| Royalties And Sublicense Fees [Member]
Other Commitments [Line Items]
Contractual obligation						$ 100,000
Rockville, Maryland [Member]
Other Commitments [Line Items]
Lease term commenced date			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			7 years 6 months
Lease expiration date			Sep. 30, 2023
Options to extend the lease			5 years
Monthly lease payments commencement period			2016-09
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

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases (Detail) $ in Thousands	Mar. 31, 2018 USD ($)
Operating Leases Future Minimum Payments Due [Abstract]
2018 (remainder of year)	$ 1,714
2019	2,296
2020	2,309
2021	1,719
2022	621
Thereafter	479
Total minimum lease payments	$ 9,138

Commitments and Contingencies - Summary of Expenses Incurred by Company (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
GlaxoSmithKline LLC [Member]
Other Commitments [Line Items]
Total related party transaction expense	$ 2,426	$ 191
GlaxoSmithKline LLC [Member] \| Sublicense Fees [Member]
Other Commitments [Line Items]
Total related party transaction expense	2,033	46
GlaxoSmithKline LLC [Member] \| Maintenance of Licensed Patents [Member]
Other Commitments [Line Items]
Total related party transaction expense	393	145
Regents of the University of Minnesota [Member]
Other Commitments [Line Items]
Total related party transaction expense	387	6
Regents of the University of Minnesota [Member] \| Sublicense Fees [Member]
Other Commitments [Line Items]
Total related party transaction expense	375
Regents of the University of Minnesota [Member] \| Maintenance of Licensed Patents [Member]
Other Commitments [Line Items]
Total related party transaction expense	12	6
License Fees [Member] \| The Trustees of the University of Pennsylvania [Member]
Other Commitments [Line Items]
Total related party transaction expense	18	130
License Fees [Member] \| The Trustees of the University of Pennsylvania [Member] \| Sublicense Fees [Member]
Other Commitments [Line Items]
Total related party transaction expense		46
License Fees [Member] \| The Trustees of the University of Pennsylvania [Member] \| Maintenance of Licensed Patents [Member]
Other Commitments [Line Items]
Total related party transaction expense	$ 18	$ 84

License Revenue - Addtional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended
	Jan. 31, 2018	Mar. 31, 2014	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
License Revenue [Line Items]
Milestone fee upon commencement of clinical trials in humans			$ 26,100
Milestone fee upon submission of regulatory approval filings			46,000
Milestone fee upon approval of commercial products by regulatory agencies			105,500
Milestone fee upon achievement of specified sales targets for licensed products			212,000
License revenue			132,391	$ 455
Interest income from licensing			1,355
Accounts receivable, current			25,976		$ 473
Accounts receivable, non -current			32,645
Other assets			883		$ 862
Contract liabilities			0
Impirment losses on receivables or contract assets			0
Contract Assets [Member]
License Revenue [Line Items]
Other assets			300
Licence Agreement [Member]
License Revenue [Line Items]
License revenue			300
Interest income from licensing			200
Accounts receivable, current			26,000
Accounts receivable, non -current			32,600
March 2014 License Agreement [Member]
License Revenue [Line Items]
License revenue				$ 100
Up-front fee paid			2,000
Transaction price of license			3,500
March 2014 License Agreement [Member] \| AveXis, Inc. [Member]
License Revenue [Line Items]
License revenue			132,100
Interest income from licensing			1,200
Accounts receivable, current			25,400
Accounts receivable, non -current			28,100
Up-front fee paid		$ 2,000
Milestone fee payments upon achievement of various development and commercialization		$ 12,300
License agreement amendment date		2018-01
Accounts receivable			53,500
January 2018 Amendment Agreement [Member]
License Revenue [Line Items]
License fee	$ 80,000
Milestone payment unpaid in the event of change of control	40,000
Transaction price of license			132,100
Sales-based milestone payments exclude transaction price			$ 120,000
January 2018 Amendment Agreement [Member] \| Minimum [Member]
License Revenue [Line Items]
Milestone payment	120,000
January 2018 Amendment Agreement [Member] \| First Anniversary [Member]
License Revenue [Line Items]
License fee	30,000
January 2018 Amendment Agreement [Member] \| Second Anniversary [Member]
License Revenue [Line Items]
License fee	$ 30,000
January 2018 Amendment Agreement [Member] \| AveXis, Inc. [Member]
License Revenue [Line Items]
License agreement period	14 years

License Revenue - Under Topic 606 (Detail) - January 2018 Amendment Agreement [Member] $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2018-01-01
License Revenue [Line Items]
Revenue expected to be recognized as of March 31, 2018	$ 80.0
Revenue, remaining performance obligation, expected timing of satisfaction, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2019-01-01
License Revenue [Line Items]
Revenue expected to be recognized as of March 31, 2018	$ 30.0
Revenue, remaining performance obligation, expected timing of satisfaction, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2020-01-01
License Revenue [Line Items]
Revenue expected to be recognized as of March 31, 2018	$ 30.0
Revenue, remaining performance obligation, expected timing of satisfaction, period	1 year

Stock-based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
	Jan. 31, 2018	Mar. 31, 2018	Mar. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense		$ 37,600
Unrecognized stock-based compensation, weighted-average period		3 years
Proceeds from exercise of stock options		$ 3,824	$ 161
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		2,429,185
Weighted-average fair values of options granted		$ 21.42
Exercise of stock options, Shares		586,694
Total intrinsic value of options exercised		$ 13,700
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		186,752
Common stock shares issued to participants		19,528

Stock-based Compensation - Stock-Based Compensation Expense by Award Type (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 3,291	$ 2,591
Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	3,122	2,439
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	68	69
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 101	$ 83

Stock-based Compensation - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 3,291	$ 2,591
Research and Development Costs [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	1,530	1,238
General and Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 1,761	$ 1,353

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	3 Months Ended	12 Months Ended
	Mar. 31, 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	914
Shares, Exercised	(587)
Shares, Cancelled or forfeited	(169)
Shares Outstanding, Ending Balance	5,626	5,468
Shares, Exercisable	2,825
Shares, Vested and expected to vest	5,626
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	32.07
Weighted-average Exercise Price, Exercised	6.57
Weighted-average Exercise Price, Cancelled or forfeited	15.35
Weighted-average Exercise Price, Outstanding, Ending Balance	14.03	$ 10.25
Weighted-average Exercise Price, Exercisable	7.42
Weighted-average Exercise Price, Vested and expected to vest	$ 14.03
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average Remaining Contractual Life (Years) Outstanding	8 years	7 years 10 months 24 days
Weighted-average Remaining Contractual Life (Years), Exercisable	7 years 3 months 18 days
Weighted-average Remaining Contractual Life (Years), Vested and expected to vest	8 years
Aggregate Intrinsic Value Outstanding	$ 91,837	$ 125,738
Aggregate Intrinsic Value, Exercisable	63,385
Aggregate Intrinsic Value, Vested and expected to vest	$ 91,837

Stock-based Compensation - Summary of Unvested RSUs Activity Under 2015 Plan (Detail) - Restricted Stock Units [Member] shares in Thousands	3 Months Ended
	Mar. 31, 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

Related Party Transactions - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2015	Dec. 31, 2014
FOXKISER,LLP [Member] \| Service Agreements [Member] \| Research and Development Costs [Member]
Related Party Transaction [Line Items]
Related party transaction expense	$ 0.5	$ 0.4
Scientific Founder and Special Advisor [Member]
Related Party Transaction [Line Items]
Related party transaction expenses from transactions with related party advisory fees		$ 0.1
Shares, Granted			211,600	211,600

Net Income (Loss) Per Share - Computations of Basic and Diluted Net Income (Loss) Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic net income (loss) per share:
Net income (loss) applicable to common stockholders	$ 104,239	$ (21,993)
Shares used in computation:
Basic	31,632	26,673
Basic	$ 3.30	$ (0.82)
Diluted net income (loss) per share:
Net income (loss) applicable to common stockholders	$ 104,239	$ (21,993)
Shares used in computation:
Weighted-average diluted common shares	34,275	26,673
Diluted	$ 3.04	$ (0.82)
Common Shares [Member]
Shares used in computation:
Weighted-average diluted common shares	31,632	26,673
Stock Option [Member]
Shares used in computation:
Weighted-average diluted common shares	2,620
Restricted Stock Units [Member]
Shares used in computation:
Weighted-average diluted common shares	22
Employee Stock Purchase Plan [Member]
Shares used in computation:
Weighted-average diluted common shares	1

Net Income (Loss) Per Share - Schedules for Computation of Diluted Weighted-Average Shares Outstanding (Detail) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	1,582	5,780
Stock Options Issued and Outstanding [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	1,582	5,724
Unvested Restricted Stock Units Outstanding [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding		40
Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding		16

Supplemental Disclosures - Schedules of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued personnel costs	$ 3,771	$ 5,789
Accrued external research and development expenses	2,851	2,072
Accrued licensing costs	1,555
Accrued external general and administrative expenses	1,184	1,078
Accrued purchases of property and equipment	86	430
Other accrued expenses and current liabilities	422	236
Accrued expenses and other current liabilities	$ 9,869	$ 9,605