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Document and Entity Information - shares	9 Months Ended
Sep. 30, 2016	Nov. 04, 2016
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2016
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	Q3
Trading Symbol	RGNX
Entity Registrant Name	REGENXBIO Inc.
Entity Central Index Key	0001590877
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		26,475,379

Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Current assets
Cash and cash equivalents	$ 28,108	$ 54,116
Marketable securities	63,662	60,025
Accounts receivable	679	2,136
Prepaid expenses	2,171	1,020
Other current assets	2,000	851
Total current assets	96,620	118,148
Marketable securities	93,087	102,226
Property and equipment, net	5,804	538
Cost method investments		300
Restricted cash	225
Other assets	239	168
Total assets	195,975	221,380
Current liabilities
Accounts payable	5,376	1,014
Accrued expenses and other current liabilities	9,006	3,198
Advance payments		127
Total current liabilities	14,382	4,339
Deferred rent, net of current portion	1,367	233
Total liabilities	15,749	4,572
Commitments and contingencies (Note 6)
Stockholders’ equity
Preferred stock; $0.0001 par value; 10,000 shares authorized, and no shares issued and outstanding at September 30, 2016 and December 31, 2015
Common stock; $0.0001 par value; 100,000 shares authorized at September 30, 2016 and December 31, 2015; 26,475 and 26,313 shares issued and outstanding at September 30, 2016 and December 31, 2015, respectively	3	3
Additional paid-in capital	274,349	269,144
Accumulated other comprehensive income (loss)	853	(719)
Accumulated deficit	(94,979)	(51,620)
Total stockholders’ equity	180,226	216,808
Total liabilities and stockholders’ equity	$ 195,975	$ 221,380

Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Revenues
License revenue	$ 65	$ 65	$ 2,638	$ 635
License revenue from related party		1,000		2,000
Reagent sales	47	61	213	200
Grant revenue	13	14	42	305
Total revenues	125	1,140	2,893	3,140
Costs of revenues
Licensing costs (including amounts to related parties)	13	213	528	527
Costs of reagent sales (including amounts to related parties)	22	44	101	94
Research and development (including amounts to related parties)	12,560	5,664	29,423	12,471
General and administrative (including amounts to related parties)	6,200	2,567	17,848	7,671
Other operating expenses (income)	(2)	(1)	(136)	15
Total operating expenses	18,793	8,487	47,764	20,778
Loss from operations	(18,668)	(7,347)	(44,871)	(17,638)
Other Income (Expense)
Investment income	514	15	1,512	23
Interest expense				(20)
Total other income (expense)	514	15	1,512	3
Net loss	(18,154)	(7,332)	(43,359)	(17,635)
Other Comprehensive Income (Loss)
Unrealized gain (loss) on available-for-sale securities	332	(26)	1,572	(26)
Total other comprehensive income (loss)	332	(26)	1,572	(26)
Comprehensive loss	(17,822)	(7,358)	(41,787)	(17,661)
Reconciliation of net loss to net loss applicable to common stockholders
Net loss	(18,154)	(7,332)	(43,359)	(17,635)
Net accretion and dividends on convertible preferred stock				(1,747)
Net gain on extinguishment of convertible preferred stock				759
Net loss applicable to common stockholders	$ (18,154)	$ (7,332)	$ (43,359)	$ (18,623)
Basic and diluted net loss per common share	$ (0.69)	$ (1.52)	$ (1.64)	$ (5.48)
Weighted-average basic and diluted common shares	26,469	4,809	26,386	3,397

Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
Statements of Cash Flows - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015
Cash flows from operating activities
Net loss	$ (43,359)	$ (17,635)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	5,031	2,059
Net amortization of premiums and accretion of discounts on marketable debt securities	1,502	1
Depreciation and amortization	264	43
Realized gains on sales of marketable securities	(20)
Unrealized foreign currency transaction gains	(2)	(4)
Imputed interest on related party promissory notes		13
Changes in operating assets and liabilities
Accounts receivable	1,459	1,277
Prepaid expenses	(1,151)	(1,372)
Other current assets	(1,149)	(127)
Other assets	(71)	(128)
Accounts payable	3,595	317
Accrued expenses and other current liabilities	4,591	758
Due to related party under services agreement		(34)
Other related party payables		(3,412)
Advance payments	(127)	(26)
Deferred rent	1,302	198
Net cash used in operating activities	(28,135)	(18,072)
Cash flows from investing activities
Restricted cash	(225)
Purchases of marketable securities	(32,262)	(19,065)
Maturities of marketable securities	38,131
Sales of marketable securities	23
Purchases of property and equipment	(3,714)	(394)
Net cash provided by (used in) investing activities	1,953	(19,459)
Cash flows from financing activities
Proceeds from exercise of stock options	174	100
Proceeds from initial public offering of common stock, net of underwriting discounts and commissions		148,233
Issuance costs for initial public offering		(618)
Net cash provided by financing activities	174	241,734
Net increase (decrease) in cash and cash equivalents	(26,008)	204,203
Cash and cash equivalents
Beginning of period	54,116	1,121
End of period	28,108	205,324
Supplemental cash flow information
Cash paid for interest		7
Supplemental disclosures of non-cash investing and financing activities
Purchases of property and equipment in accounts payable and accrued expenses	$ 1,816	34
Issuance costs for initial public offering in accounts payable and accrued expenses		2,431
Conversion of related party promissory notes into Series C convertible preferred stock		1,389
Conversion of convertible preferred stock into common stock upon initial public offering		111,392
Series C Convertible Preferred Stock [Member]
Cash flows from financing activities
Proceeds from issuance of convertible preferred stock, net of transaction costs		26,021
Supplemental disclosures of non-cash investing and financing activities
Conversion of accrued service fees to related party		2,403
Series D Convertible Preferred Stock [Member]
Cash flows from financing activities
Proceeds from issuance of convertible preferred stock, net of transaction costs		$ 67,998

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2016

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation and Unaudited Interim Financial Information

The accompanying 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 financial statements have been prepared on the same basis as the annual audited financial statements as of and for the year ended December 31, 2015 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2015, filed with the Securities and Exchange Commission (SEC) on March 3, 2016. Certain information and footnote disclosures required by GAAP which are normally included in the Company’s annual financial statements have been condensed or omitted pursuant to SEC rules and regulations for interim reporting. In the opinion of management, the accompanying financial statements reflect all adjustments, which include all normal and recurring adjustments necessary for the fair statement of the Company’s financial position as of September 30, 2016, and the results of its operations and its cash flows for the interim periods ended September 30, 2016 and 2015.

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 financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2015, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K.

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 materially differ 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 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 financial statements. Estimates are used in the following areas, among others: stock-based compensation expense, accrued research and development expenses and the fair value of financial instruments.

Restricted Cash

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.

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.

Net Loss Per Share

The Company computes net loss per share in conformity with the two-class method required for participating securities. The Company considers all series of convertible preferred stock outstanding prior to the IPO to be participating securities. The holders of convertible preferred stock outstanding prior to the IPO were entitled to receive preferential dividends in the event that a dividend was to be paid to the holders of common stock, and did not have a contractual obligation to share in the losses of the Company. As such, the Company’s net losses for the three and nine months ended September 30, 2015 were not allocated to these participating securities. In connection with the IPO, all outstanding shares of convertible preferred stock were automatically converted into shares of common stock.

Basic net loss per share is calculated by dividing net loss applicable to holders of common stock by the weighted-average shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is calculated by adjusting weighted-average shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury-stock method. For purposes of the diluted net loss per share calculation, convertible preferred stock, outstanding stock options and withholdings under the employee stock purchase plan are considered to be common stock equivalents but have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive. Contingently convertible shares in which conversion is based on non-market-priced contingencies are excluded from the calculations of both basic and diluted net loss per share until the contingency has been fully met. Accordingly, basic and diluted net loss per share were the same for all periods presented.

Recently Announced Accounting Pronouncements

In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes the revenue recognition requirements in ASC 605, Revenue Recognition. This ASU is based on the principle that revenue is recognized to depict the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The ASU also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. In August 2015, the FASB issued ASU No. 2015-14, Revenue from Contracts with Customers (Topic 606), which deferred the effective date of the guidance under ASU No. 2014-09 by one year. In April 2016, the FASB issued ASU 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing, which clarifies various aspects of Topic 606, including the identification of performance obligations and the implementation of licensing guidance. In May 2016, the FASB issued ASU 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients, which clarifies various additional aspects of Topic 606, including the assessment of collectability, presentation of sales taxes and other similar taxes collected from customers, the measurement date for transactions with non-cash consideration as well as transitional issues and other technical corrections regarding the adoption of new standards under Topic 606. The standards are effective for annual and interim reporting periods beginning after December 15, 2017. Earlier application is permitted only as of annual reporting periods beginning after December 15, 2016, including interim reporting periods within that reporting period. The Company is evaluating the application of these ASU’s, but has not yet determined the potential effects they may have on the Company’s financial statements.

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 to 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 March 2016, the FASB issued ASU 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting, which is intended to simplify several aspects of the accounting for share-based payment awards including income tax consequences, classification of awards as either equity or liabilities and classification within the statement of cash flows. The standard is effective for annual and interim periods beginning after December 15, 2016, with early adoption permitted upon issuance. The Company does not believe the application of this standard will have a material impact on the Company’s 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 ASU, but has not yet determined the potential effects it may have on the Company’s financial statements.

In January 2016, the FASB issued ASU 2016-01, Financial Instruments—Overall (Topic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, which supersedes the current guidance to classify equity securities with readily determinable fair values into different categories and requires equity securities to be measured at fair value with changes in the fair value recognized through net income (loss). The standard is effective for annual and interim periods beginning after December 15, 2017. 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 August 2014, the FASB issued ASU 2014-15, Presentation of Financial Statements—Going Concern (Subtopic 205-40): Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern, requiring management to evaluate whether events or conditions could impact an entity’s ability to continue as a going concern and to provide disclosures if necessary. Management will be required to perform the evaluation within one year after the date that the financial statements are issued. Disclosures will be required if conditions give rise to substantial doubt and the type of disclosure will be determined based on whether management’s plans will be able to alleviate the substantial doubt. The standard is effective for the first annual period ending after December 15, 2016, and for annual periods and interim periods thereafter with early application permitted. The Company does not believe that, upon its adoption, the application of this standard will have a material impact on the Company’s financial statement disclosures.

Marketable Securities

9 Months Ended

Sep. 30, 2016

Investments Debt And Equity Securities [Abstract]

Marketable Securities

3. Marketable Securities

The following table presents a summary of the Company’s marketable securities, which consist solely of available-for-sale securities:

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
September 30, 2016
Corporate bonds	$	155,596	$	292	$	(34	)	$	155,854
Common equity securities		300		595		—			895
	$	155,896	$	887	$	(34	)	$	156,749

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
December 31, 2015
Corporate bonds	$	157,977	$	4	$	(759	)	$	157,222
Commercial paper		4,990		—		—			4,990
Common equity securities		3		36		—			39
	$	162,970	$	40	$	(759	)	$	162,251

As of December 31, 2015, the Company’s common equity securities consisted of shares of common stock of Dimension Therapeutics, Inc. (Dimension), which became a publicly traded company in October 2015. The Company obtained these shares in connection with a license granted to Dimension in October 2013. As of September 30, 2016, the Company had sold all of its shares of Dimension common stock.

As of September 30, 2016, the Company’s common equity securities consisted of shares of common stock of Audentes Therapeutics, Inc. (Audentes), which became a publicly traded company in July 2016. The Company obtained these shares in connection with a license granted to Audentes in July 2013. The Company is restricted from trading these securities until January 2017 pursuant to a lock-up agreement entered into in connection with Audentes’ IPO. The Company has classified these shares as available-for-sale securities and recognized an unrealized gain of $595 which is included in accumulated other comprehensive income as of September 30, 2016. Prior to Audentes’ IPO, the shares were not marketable and were accounted for as a cost method investment on the Company’s balance sheets.

As of September 30, 2016 and December 31, 2015, no available-for-sale securities had remaining maturities greater than three years.

The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. As of September 30, 2016 and December 31, 2015, the balance in the Company’s accumulated other comprehensive income (loss) consisted solely of net unrealized gains and losses on available-for-sale securities. For the nine months ended September 30, 2016, the Company recognized net unrealized gains on available-for-sale securities of $1,572, which is included in other comprehensive income for the period. The Company recognized realized gains of $20 on the sale or maturity of available-for-sale securities during the nine months ended September 30, 2016, which were reclassified out of accumulated other comprehensive income (loss) during the period. The realized gains on available-for-sale securities related solely to the sale of Dimension common stock during the period.

The aggregate fair value of securities held by the Company in an unrealized loss position for less than 12 months as of September 30, 2016 and December 31, 2015 was $40,512 and $155,486, respectively. The Company did not hold any securities in an unrealized loss position for more than 12 months as of December 31, 2015. The aggregate fair value of securities held by the Company in an unrealized loss position for more than 12 months as of September 30, 2016 was $4,116. The aggregate unrealized loss for those securities in an unrealized loss position for more than 12 months as of September 30, 2016 was $6. As of September 30, 2016, securities held by the Company which were in an unrealized loss position consisted of eleven investment grade corporate bonds. The Company has the intent and ability to hold such securities until recovery and has determined that none of its investments were other-than-temporarily impaired as of September 30, 2016 or December 31, 2015.

- Definition

The entire disclosure for investments in certain debt and equity securities.

+ References + Details

Fair Value of Financial Instruments

9 Months Ended

Sep. 30, 2016

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 solely of money market mutual funds. Marketable securities consist of corporate debt securities, including corporate bonds and commercial paper, as well as common equity securities as disclosed in Note 3. The following tables present the fair value of cash equivalents and marketable securities in accordance with the hierarchy discussed in Note 2:

	Quoted		Significant
	prices		other		Significant
	in active		observable		unobservable
	markets		inputs		inputs
	(Level 1)		(Level 2)		(Level 3)		Total
September 30, 2016
Money market mutual funds (cash equivalents)	$	—	$	28,108	$	—	$	28,108
Corporate bonds (marketable securities)		—		155,854		—		155,854
Common equity securities (marketable securities)		895		—		—		895
	$	895	$	183,962	$	—	$	184,857

	Quoted		Significant
	prices		other		Significant
	in active		observable		unobservable
	markets		inputs		inputs
	(Level 1)		(Level 2)		(Level 3)		Total
December 31, 2015
Money market mutual funds (cash equivalents)	$	—	$	54,104	$	—	$	54,104
Corporate bonds (marketable securities)		—		157,222		—		157,222
Commercial paper (marketable securities)		—		4,990		—		4,990
Common equity securities (marketable securities)		39		—		—		39
	$	39	$	216,316	$	—	$	216,355

Management estimates that the carrying amounts of its accounts receivable, accounts payable and accrued expenses and other current liabilities approximate fair value due to the short-term nature of those instruments.

The Company has determined that it is not practicable to estimate the fair value of cost method investments. The Company has not identified any events or changes in circumstances that would have an adverse effect on the fair value of its cost method investments reported as of December 31, 2015.

- Definition

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

+ References + Details

Commitments and Contingencies

9 Months Ended

Sep. 30, 2016

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 which 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 5.5-year, non-cancelable operating lease for office space in Rockville, Maryland. The lease commenced in April 2015, and expires in September 2020. The Company has options to extend the lease for up to 6 years. Initial monthly payments required under the lease were $24 beginning in October 2015 and escalate annually in accordance with the lease.

In September 2015, and again in November 2015, the Company amended its operating lease in Rockville, Maryland to include additional office and laboratory space and extend the term of the lease for its existing space to October 2020. The lease for the additional space commenced in April 2016, and has a 5-year term expiring in March 2021. The Company has options to extend the lease for the additional space to be coterminous with the Company’s existing lease at that facility. Initial monthly payments required under the lease for the additional space were $41 and escalate annually in accordance with the lease. The Company received a $286 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 January 2016, the Company entered into a 7.5-year, non-cancelable operating lease for additional office space in Rockville, Maryland. The lease commenced in February 2016, and expires in September 2023. Initial monthly payments required under the lease are $38 beginning seven months from the commencement date and escalate annually in accordance with the lease agreement. The Company received a $725 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 in New York, New York. The lease commenced in July 2016, and expires in October 2020. Initial monthly payments required under the lease are $25 beginning three months from the commencement date 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 $225 which the landlord may draw upon in the event of any uncured default by the Company under the terms of the lease. As of September 30, 2016, the Company has recorded restricted cash of $225 as collateral to the financial institution which issued the letter of credit.

The Company entered into a short-term operating lease for office space in Gaithersburg, Maryland which expired in October 2016.

As of September 30, 2016, future minimum lease payments under non-cancelable operating leases are as follows:

	Operating
	Leases
2016 (remainder of year)	$	387
2017		1,589
2018		1,637
2019		1,685
2020		1,611
Thereafter		1,614
Total minimum lease payments	$	8,523

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 statements of operations and comprehensive loss. Sublicense fees are based on a specified percentage of license fees earned by the Company and are included in licensing costs in the statements of operations and comprehensive loss. Royalties on sales of licensed reagents for use in research and development are included in costs of reagent sales in the statements of operations and comprehensive 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, as amended, with The Trustees of 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 24.5 percent of the then outstanding membership interest in the LLC 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 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:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2016		2015		2016		2015
Sublicense fees	$	7	$	107	$	264	$	264
Royalties on sales of reagents		2		2		11		8
Maintenance of licensed patents		42		29		85		118
	$	51	$	138	$	360	$	390

As of September 30, 2016 and December 31, 2015, the Company had accrued $22 and $440, respectively, in expenses payable to Penn under the license agreement, which are included in accounts payable and accrued expenses on the Company’s balance sheets. Until September 30, 2015, the Company considered Penn to be a related party. See Note 9 for further information on related party transactions with Penn.

GlaxoSmithKline LLC. In March 2009, the Company entered into a license agreement, as amended, 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 19.9 percent of the then outstanding membership interest in the LLC 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 obligated to pay GSK up to $1,500 upon the achievement of various milestones. As of September 30, 2016, no milestones have been achieved, or deemed probable of achievement, and accordingly no milestone payments were payable to GSK.

Expenses incurred by the Company related to its license from GSK were as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2016		2015		2016		2015
Sublicense fees	$	7	$	107	$	264	$	264
Royalties on sales of reagents		1		1		7		5
Maintenance of licensed patents		89		57		319		474
	$	97	$	165	$	590	$	743

As of September 30, 2016 and December 31, 2015, the Company had accrued $74 and $526, respectively, in expenses payable to GSK under the license agreement, which are included in accounts payable and accrued expenses on the Company’s balance sheets. Until September 30, 2015, the Company considered GSK to be a related party. See Note 9 for further information on related party transactions with GSK.

ARIAD Pharmaceuticals, Inc. In November 2010, the Company entered into a license agreement with ARIAD Pharmaceuticals, Inc. (ARIAD), for exclusive, worldwide rights to certain patents owned and exclusively licensed by ARIAD. In consideration for the license, the Company issued Class A Units of the LLC to ARIAD with a fair value of $726. Under the terms of the agreement, the Company is obligated to pay ARIAD royalties on net sales, and sublicense fees, if any. Additionally, the Company is obligated to pay ARIAD up to $2,300 and annual maintenance fees of $50 upon the achievement of various milestones. As of September 30, 2016, no milestones have been achieved, or deemed probable of achievement, and accordingly no milestone payments or maintenance fees were payable to ARIAD. Additionally, the Company has not incurred any royalties or sublicense fees payable to ARIAD since the inception of the agreement. There were no amounts due to ARIAD under the agreement as of September 30, 2016 and December 31, 2015.

Regents of the University of Minnesota. In November 2014, the Company entered into a license agreement 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 of $25 and reimbursed Minnesota for patent maintenance expenses of $9. Under the terms of the agreement, the Company is obligated to pay Minnesota annual maintenance fees between $5 and $15 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 $125 per licensed product upon the achievement of various milestones. As of September 30, 2016, no milestones have been achieved, or deemed probable of achievement, and accordingly no milestone payments were payable to Minnesota. Additionally, the Company has not incurred any royalties or sublicense fees payable to Minnesota since the inception of the agreement. As of September 30, 2016 and December 31, 2015, the Company had accrued $25 and $0, respectively, in patent maintenance expenses payable to Minnesota under the license agreement.

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 exposure under these agreements is unknown because it involves claims that may be made against the Company in the future. To date, the Company has not paid any claims or been required to defend any action related to its indemnification obligations. As of September 30, 2016 and December 31, 2015, the Company did not have any material indemnification claims that were probable or reasonably possible and consequently has not recorded any related liabilities.

Significant Agreements

9 Months Ended

Sep. 30, 2016

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Significant Agreements

7. Significant Agreements

See Note 6 for license agreements granted to the Company.

Licenses Granted by the Company

The Company has granted a number of intellectual property licenses 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. License agreements generally have a term equal to the life of the underlying patents and are terminable only at the option of the licensee. License agreements may require licensees to pay non-refundable up-front fees, option fees and annual maintenance fees. Additional contingent consideration under the licenses may include sublicense fees, milestone fees and royalties on net sales of commercialized 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-teen percentage of net sales by licensees.

Milestone fees are payable to the Company upon the achievement of specific clinical and regulatory developments by licensees. As of September 30, 2016, 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 $500 upon the submission of preclinical regulatory filings, $23,650 upon the commencement of various stages of clinical trials, $37,000 upon the submission of regulatory approval filings, $91,500 upon the approval of commercial products by regulatory agencies and $92,000 upon the achievement of specified sales targets for licensed products.

License revenue consists of the following:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2016		2015		2016		2015
Up-front fees and option fees for commercial licenses	$	—	$	1,000	$	2,000	$	2,000
Maintenance fees for commercial licenses		65		65		395		245
Milestone fees		—		—		—		250
Research and other license revenue		—		—		243		140
	$	65	$	1,065	$	2,638	$	2,635

Stock-based Compensation

9 Months Ended

Sep. 30, 2016

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-based Compensation

8. Stock-based Compensation

Stock-based Compensation Expense

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

	Three Months Ended September 30,				Nine Months Ended September 30,
	2016		2015		2016		2015
Stock options	$	1,800	$	1,349	$	5,002	$	2,059
Employee stock purchase plan		29		—		29		—
	$	1,829	$	1,349	$	5,031	$	2,059

The Company has recorded aggregate stock-based compensation expense in the statement of operations and comprehensive loss as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2016		2015		2016		2015
Research and development	$	814	$	1,101	$	1,824	$	1,413
General and administrative		1,015		248		3,207		646
	$	1,829	$	1,349	$	5,031	$	2,059

Stock Options

In September 2014, the Board of Directors adopted the 2014 Stock Plan (2014 Plan). In June 2015, the Board of Directors adopted the 2015 Equity Incentive Plan (2015 Plan), which became effective on September 16, 2015, the date on which the registration statement for the IPO was declared effective. The 2015 Plan replaced the 2014 Plan, and as of the effective date of the 2015 Plan, no further awards may be issued under the 2014 Plan. Any options or awards outstanding under the 2014 Plan as of the effective date of the 2015 Plan remained outstanding and effective. The initial amount of shares authorized for issuance under the 2015 Plan was 2,952. The number of authorized shares under the 2015 Plan automatically increases annually on January 1, beginning January 1, 2016, by the lesser of (i) 4% of the total number of shares of common stock outstanding on December 31 of the prior year, or (ii) a number of common shares determined by the Board of Directors. Effective January 1, 2016, the Board of Directors authorized an additional 1,053 shares to be issued under the 2015 Plan. As of September 30, 2016, the total number of shares of common stock authorized for issuance under the 2015 Plan and 2014 Plan is 7,178, of which 1,848 remain available for future grants under the 2015 Plan.

The 2014 Plan and 2015 Plan provide for the issuance of stock options, stock appreciation rights, restricted and unrestricted stock awards and performance cash awards to employees, members of the Board of Directors and consultants of the Company. No stock appreciation rights, restricted or unrestricted stock awards or performance cash awards have been granted under the 2014 Plan and 2015 Plan since the inception of the plans. Stock options under the 2014 Plan and 2015 Plan generally expire ten years following the date of grant. Options typically vest over a four year period, but vesting provisions can vary by award based on the discretion of the Board of Directors. Certain awards issued by the Company include performance conditions that must be achieved in order for vesting to occur. Stock options under the 2014 Plan and 2015 Plan carry an exercise price at least equal to the estimated fair value of the Company’s common stock on the date of grant.

Shares of common stock underlying awards previously issued under the 2014 Plan and 2015 Plan which are reacquired by the Company, withheld by the Company in payment of the purchase price, exercise price or withholding taxes, expired, cancelled due to forfeiture or otherwise terminated other than by exercise, are added to the number of shares of common stock available for issuance under the 2015 Plan. Shares available for issuance under the 2015 Plan may be either authorized but unissued shares of the Company’s common stock or common stock reacquired by the Company and held in treasury. The 2015 Plan expires in June 2025, ten years from the date it was adopted by the Board of Directors, unless earlier terminated.

The following table summarizes stock option activity under the 2014 Plan and 2015 Plan:

						Weighted-
						average
				Weighted-		Remaining
				average		Contractual		Aggregate
				Exercise		Life		Intrinsic
	Shares			Price		(Years)		Value (a)
Outstanding at December 31, 2015		3,684		$	5.52		9.1	$	44,472
Granted		1,696		$	12.30
Exercised		(162	)	$	1.08
Cancelled or forfeited		(178	)	$	14.62
Outstanding at September 30, 2016		5,040		$	7.62		8.7	$	58,033
Exercisable at September 30, 2016		1,898		$	3.26		8.3	$	23,110
Vested and expected to vest at September 30, 2016		5,040		$	7.62		8.7	$	58,033

(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 of options granted during the nine months ended September 30, 2016 was $8.18. During the nine months ended September 30, 2016, the total number of stock options exercised was 162, resulting in total proceeds of $174. The total intrinsic value of options exercised during the nine months ended September 30, 2016 was $1,692.

Stock Options Granted to Employees. The fair value of options granted to employees was estimated at the date of grant using the Black-Scholes valuation model with the following weighted-average assumptions:

	Nine Months Ended September 30,
	2016			2015
Expected volatility		75	%		67	%
Expected term (in years)		6.2			6.1
Risk-free interest rate		1.5	%		1.7	%
Expected dividend yield		0.0	%		0.0	%

As of September 30, 2016, there was $19,003 of unrecognized stock-based compensation expense related to stock option awards to employees, which is expected to be recognized over a weighted-average period of 3.23 years.

Stock Options Granted to Non-employees. Stock-based compensation expense related to stock options granted to non-employees is recognized as the stock options are earned. The Company believes that the estimated fair value of the stock options is more readily measurable than the fair value of the services rendered. The Company used the following weighted-average assumptions in estimating non-employees’ stock-based compensation expense:

	Nine Months Ended September 30,
	2016			2015
Expected volatility		83	%		71	%
Expected term (in years)		8.1			9.7
Risk-free interest rate		1.6	%		2.1	%
Expected dividend yield		0.0	%		0.0	%

Employee Stock Purchase Plan

In June 2015, the Company’s Board of Directors adopted the 2015 Employee Stock Purchase Plan (2015 ESPP), which became effective on September 16, 2015, the date on which the registration statement for the IPO was declared effective. The 2015 ESPP authorizes the initial issuance of up to a total of 254 shares of the Company’s common stock to participating employees. The number of shares reserved for issuance under the 2015 ESPP automatically increases on the first business day of each fiscal year, commencing in 2016, by a number equal to the lesser of (i) 1% of the shares of common stock outstanding on the last business day of the prior fiscal year; or (ii) the number of shares determined by the Company’s Board of Directors. Unless otherwise determined by the administrator of the 2015 ESPP, two offering periods of six months’ duration will begin each year on January 1 and July 1. The Board of Directors did not authorize any further shares to be reserved for issuance under the 2015 ESPP in January 2016. As of September 30, 2016, no shares have been issued under the 2015 ESPP.

- Definition

The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details.

+ References + Details

Related Party Transactions	9 Months Ended
Related Party Transactions	Sep. 30, 2016
Related Party Transactions [Abstract]
Related Party Transactions	9. Related Party Transactions The Trustees of the University of Pennsylvania Penn received an equity interest in the Company as consideration for the intellectual property license granted to the Company in February 2009, and prior to September 30, 2015, the Company considered Penn to be a related party. See Note 6 for further information, including costs incurred by the Company, regarding its license from Penn. In addition to the license agreement, Penn also provides manufacturing and research and development services to the Company. Penn was considered a related party of the Company for the three and nine months ended September 30, 2015. During the three and nine months ended September 30, 2015, the Company incurred $40 and $80, respectively, in services from Penn for the manufacturing of reagents for sale by the Company as well as $2,493 and $5,136, respectively, in research and development services provided by Penn. As a result of various factors, including the dilution to the Company’s stockholders resulting from the sale and issuance of Series C convertible preferred stock (Series C Preferred Stock) and Series D convertible preferred stock (Series D Preferred Stock), and the closing of the Company’s IPO on September 22, 2015, the Company no longer considers Penn to be a related party subsequent to the period ended September 30, 2015. GlaxoSmithKline LLC GSK received an equity interest in the Company as consideration for the intellectual property license granted to the Company in March 2009, and prior to September 30, 2015, the Company considered GSK to be a related party. See Note 6 for further information, including costs incurred by the Company, regarding its license from GSK. As a result of various factors, including the dilution to the Company’s stockholders resulting from the sale and issuance of Series C Preferred Stock and Series D Preferred Stock, and the closing of the Company’s IPO on September 22, 2015, the Company no longer considers GSK to be a related party subsequent to the period ended September 30, 2015. Dimension Therapeutics, Inc. The Company and a number of its members, directors and management received common stock of Dimension as consideration for a license granted to Dimension in October 2013. Additionally, at the date of the license, three of the Company’s board members also served on the board of directors of Dimension. As a result, prior to September 30, 2015, the Company considered Dimension to be a related party. As of September 30, 2015, the Company no longer had any overlapping board members with Dimension. Additionally, the Company’s directors and management do not have significant influence over Dimension. Accordingly, the Company no longer considers Dimension to be a related party subsequent to the period ended September 30, 2015. During the three and nine months ended September 30, 2015, the Company recognized license revenue of $1,000 and $2,000, respectively, from commercial licenses granted to Dimension, which is included in license revenue from related party in the statements of operations and comprehensive loss. Revenue earned and recognized from Dimension subsequent to September 30, 2015 is no longer included in revenue from related party in the statements of operations and comprehensive loss. In 2014, the Company received $200 from Dimension for the purchase of materials owned by the Company and used in the Company’s manufacturing process for research and development and clinical trials. The $200 was recognized as a gain on disposal of the material as the material was delivered to Dimension. As of September 30, 2015, the Company had recognized $73 of the total gain on disposal and had recorded a liability of $127 related to undelivered material. During the three and nine months ended September 30, 2015, the Company recognized gains of $5 and $26, respectively, for the delivery of the material to Dimension. FoxKiser The Company was party to a services agreement, as amended from time to time, with FoxKiser LLP (FoxKiser), an affiliate of certain stockholders of the Company and an affiliate of one current and one former member of the Board of Directors, which was terminated in January 2015. Under the agreement, the Company paid a fixed monthly fee plus an additional support fee, as determined by FoxKiser on a monthly basis, as consideration for office facilities, equipment, supplies, general management, accounting, financial management, human resources, legal, secretarial, regulatory compliance and other services provided to the Company. The amounts outstanding to FoxKiser under the services agreement in excess of 30 days from their due date accrued interest at 1.5 percent per month, compounding monthly. The Company allocated the service fees from FoxKiser between research and development and general and administrative expense. For the three and nine months ended September 30, 2015, costs incurred under the services agreement with FoxKiser were $0 and $345, respectively. In January 2015, the Company entered into an operating lease with FoxKiser for office space in Washington, D.C. The lease agreement, which had a month-to-month term, required monthly payments of $20. The lease was terminated in April 2015. Rent expense to FoxKiser during the three and nine months ended September 30, 2015 was $0 and $60, respectively. The Company entered into a Professional Services Agreement with FoxKiser effective as of January 1, 2016, pursuant to which the Company incurs a fixed monthly fee of $80 in consideration for certain strategic planning, development and regulatory services to be provided by FoxKiser. The Professional Services Agreement has an initial term of one year and is terminable by either party, at any time, upon sixty days’ prior written notice to the other party. Costs incurred under the Professional Services Agreement with FoxKiser for the three and nine months ended September 30, 2016 were $240 and $720, which have been recorded as research and development expenses in the statements of operations and comprehensive loss. Chief Scientific Advisor In September 2014, the Company entered into an advisory agreement with its Chief Scientific Advisor, who is also the Chairman of the Company’s Scientific Advisory Board. The agreement was amended in May 2015 and expires in March 2017. During the three months ended September 30, 2016 and 2015, the Company incurred advisory fees of $62 and $45, respectively, to the Chief Scientific Advisor under the agreement. During the nine months ended September 30, 2016 and 2015, the Company incurred advisory fees of $187 and $135, respectively, to the Chief Scientific Advisor under the agreement. Additionally, since the inception of the agreement, the Company has granted options to purchase 212 shares of common stock to the Chief Scientific Advisor as compensation for services to be provided to the Company, which vest partially upon the completion of future service conditions and partially upon the achievement of specified performance conditions as set forth in the award agreements.

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2016

Accounting Policies [Abstract]

Basis of Presentation and Unaudited Interim Financial Information

Use of Estimates

Restricted Cash

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.

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.

Net Loss Per Share

Recently Announced Accounting Pronouncements

Marketable Securities (Tables)

9 Months Ended

Sep. 30, 2016

Investments Debt And Equity Securities [Abstract]

Summary of Company Marketable Securities

The following table presents a summary of the Company’s marketable securities, which consist solely of available-for-sale securities:

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
September 30, 2016
Corporate bonds	$	155,596	$	292	$	(34	)	$	155,854
Common equity securities		300		595		—			895
	$	155,896	$	887	$	(34	)	$	156,749

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
December 31, 2015
Corporate bonds	$	157,977	$	4	$	(759	)	$	157,222
Commercial paper		4,990		—		—			4,990
Common equity securities		3		36		—			39
	$	162,970	$	40	$	(759	)	$	162,251

- 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)

9 Months Ended

Sep. 30, 2016

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:

	Quoted		Significant
	prices		other		Significant
	in active		observable		unobservable
	markets		inputs		inputs
	(Level 1)		(Level 2)		(Level 3)		Total
September 30, 2016
Money market mutual funds (cash equivalents)	$	—	$	28,108	$	—	$	28,108
Corporate bonds (marketable securities)		—		155,854		—		155,854
Common equity securities (marketable securities)		895		—		—		895
	$	895	$	183,962	$	—	$	184,857

	Quoted		Significant
	prices		other		Significant
	in active		observable		unobservable
	markets		inputs		inputs
	(Level 1)		(Level 2)		(Level 3)		Total
December 31, 2015
Money market mutual funds (cash equivalents)	$	—	$	54,104	$	—	$	54,104
Corporate bonds (marketable securities)		—		157,222		—		157,222
Commercial paper (marketable securities)		—		4,990		—		4,990
Common equity securities (marketable securities)		39		—		—		39
	$	39	$	216,316	$	—	$	216,355

Commitments and Contingencies (Tables)

9 Months Ended

Sep. 30, 2016

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

As of September 30, 2016, future minimum lease payments under non-cancelable operating leases are as follows:

	Operating
	Leases
2016 (remainder of year)	$	387
2017		1,589
2018		1,637
2019		1,685
2020		1,611
Thereafter		1,614
Total minimum lease payments	$	8,523

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:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2016		2015		2016		2015
Sublicense fees	$	7	$	107	$	264	$	264
Royalties on sales of reagents		2		2		11		8
Maintenance of licensed patents		42		29		85		118
	$	51	$	138	$	360	$	390

GlaxoSmithKline LLC [Member]

Summary of Expenses Incurred by Company

Expenses incurred by the Company related to its license from GSK were as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2016		2015		2016		2015
Sublicense fees	$	7	$	107	$	264	$	264
Royalties on sales of reagents		1		1		7		5
Maintenance of licensed patents		89		57		319		474
	$	97	$	165	$	590	$	743

Stock-based Compensation (Tables)

9 Months Ended

Sep. 30, 2016

Stock-Based Compensation Expense by Award Type

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

	Three Months Ended September 30,				Nine Months Ended September 30,
	2016		2015		2016		2015
Stock options	$	1,800	$	1,349	$	5,002	$	2,059
Employee stock purchase plan		29		—		29		—
	$	1,829	$	1,349	$	5,031	$	2,059

Stock-Based Compensation Expense

The Company has recorded aggregate stock-based compensation expense in the statement of operations and comprehensive loss as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2016		2015		2016		2015
Research and development	$	814	$	1,101	$	1,824	$	1,413
General and administrative		1,015		248		3,207		646
	$	1,829	$	1,349	$	5,031	$	2,059

Schedule of Fair Value of Options Granted to Employees Valuation Assumptions

The fair value of options granted to employees was estimated at the date of grant using the Black-Scholes valuation model with the following weighted-average assumptions:

	Nine Months Ended September 30,
	2016			2015
Expected volatility		75	%		67	%
Expected term (in years)		6.2			6.1
Risk-free interest rate		1.5	%		1.7	%
Expected dividend yield		0.0	%		0.0	%

Non Employee Stock Option [Member]

Schedule of Non-Employees Stock Purchase Plan Valuation Assumptions

The Company used the following weighted-average assumptions in estimating non-employees’ stock-based compensation expense:

	Nine Months Ended September 30,
	2016			2015
Expected volatility		83	%		71	%
Expected term (in years)		8.1			9.7
Risk-free interest rate		1.6	%		2.1	%
Expected dividend yield		0.0	%		0.0	%

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:

						Weighted-
						average
				Weighted-		Remaining
				average		Contractual		Aggregate
				Exercise		Life		Intrinsic
	Shares			Price		(Years)		Value (a)
Outstanding at December 31, 2015		3,684		$	5.52		9.1	$	44,472
Granted		1,696		$	12.30
Exercised		(162	)	$	1.08
Cancelled or forfeited		(178	)	$	14.62
Outstanding at September 30, 2016		5,040		$	7.62		8.7	$	58,033
Exercisable at September 30, 2016		1,898		$	3.26		8.3	$	23,110
Vested and expected to vest at September 30, 2016		5,040		$	7.62		8.7	$	58,033

(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.

Nature of Business - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	Sep. 22, 2015	Sep. 30, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Accumulated deficit		$ (94,979)	$ (51,620)
Cash, cash equivalents and marketable securities		$ 184,857
Common Stock [Member]
Class of Stock [Line Items]
Conversion of convertible preferred stock into common stock upon initial public offering, Shares	16,298
Initial Public Offering [Member]
Class of Stock [Line Items]
Issuance of convertible preferred stock, shares	7,245
Stock issuance price per share	$ 22.00
Initial Public Offering [Member] \| Common Stock [Member]
Class of Stock [Line Items]
Net proceeds from issuance of common stock	$ 145,184
Overallotment Option [Member]
Class of Stock [Line Items]
Issuance of convertible preferred stock, shares	945

Marketable Securities - Additional Information (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
Marketable Securities - Additional Information (Detail)	Sep. 30, 2016 USD ($) Bond	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) Bond	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Unrealized gain (loss) on available-for-sale securities	$ 332,000	$ (26,000)	$ 1,572,000	$ (26,000)
Available for sale securities remaining maturities greater than three years	0		0		$ 0
Realized gains or losses available for sale securities			20,000
Aggregate fair value of securities in an unrealized loss position for less than twelve months	40,512,000		40,512,000		155,486,000
Aggregate fair value of securities in an unrealized loss position for more than twelve months	$ 4,116,000		4,116,000
Aggregate unrealized loss position for more than twelve months			6,000
Other-than-temporary impaired			$ 0		$ 0
Number of investment grade corporate bonds \| Bond	11		11
Common Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Unrealized gain (loss) on available-for-sale securities			$ 595,000
Common Equity Securities [Member] \| Dimension Therapeutics, Inc. [Member]
Schedule Of Available For Sale Securities [Line Items]
Realized gains or losses available for sale securities			$ 20,000

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended								9 Months Ended	12 Months Ended
	May 31, 2016 USD ($)	Jan. 31, 2016 USD ($)	Oct. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Mar. 31, 2015	Nov. 30, 2014 USD ($)	Nov. 30, 2010 USD ($)	Mar. 31, 2009 USD ($)	Sep. 30, 2016 USD ($) Milestone	Dec. 31, 2015 USD ($)
Other Commitments [Line Items]
Tenant improvement allowance received				$ 286,000
Restricted cash as collateral with financial institution									$ 225,000
Claims paid to date related to indemnification issues									0
Accruals or expenses related to indemnification issues									$ 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									$ 22,000	440,000
GlaxoSmithKline LLC [Member]
Other Commitments [Line Items]
Percentage of ownership interest issued as consideration for license agreement								19.90%
Accrued expenses									$ 74,000	526,000
Number of milestone \| Milestone									0
Milestone payment obligation								$ 1,500,000	$ 0
ARIAD Pharmaceuticals, Inc [Member]
Other Commitments [Line Items]
Number of milestone \| Milestone									0
Annual maintenance fees							$ 50,000		$ 0
Milestone payments									0
Royalty expense incurred									0
Sublicense fees									0
Amounts due under agreement									$ 0	0
ARIAD Pharmaceuticals, Inc [Member] \| Maximum [Member]
Other Commitments [Line Items]
Contractual obligation							2,300,000
ARIAD Pharmaceuticals, Inc [Member] \| Common Class A [Member]
Other Commitments [Line Items]
Fair value of stock issued for license							$ 726,000
Regents of the University of Minnesota [Member]
Other Commitments [Line Items]
Number of milestone \| Milestone									0
Annual maintenance fees						$ 9,000
Milestone payments									$ 0
Amounts due under agreement									$ 25,000	$ 0
Up-front fee paid						25,000
Regents of the University of Minnesota [Member] \| Maximum [Member] \| Annual License Maintenance Fee [Member]
Other Commitments [Line Items]
Contractual obligation						15,000
Regents of the University of Minnesota [Member] \| Maximum [Member] \| Royalties And Sublicense Fees [Member]
Other Commitments [Line Items]
Contractual obligation						125,000
Regents of the University of Minnesota [Member] \| Minimum [Member] \| Annual License Maintenance Fee [Member]
Other Commitments [Line Items]
Contractual obligation						$ 5,000
Rockville, Maryland [Member]
Other Commitments [Line Items]
Lease period		7 years 6 months		5 years	5 years 6 months
Lease expiration date		Sep. 30, 2023		Mar. 31, 2021	Sep. 30, 2020
Options to extend the lease					6 years
Lease commenced date		2016-02			2015-04
Monthly lease payments		$ 38,000	$ 24,000	$ 41,000
Lease term									Extend the term of the lease for its existing space to October 2020.
Tenant improvement allowance received		$ 725,000
New York, New York [Member]
Other Commitments [Line Items]
Lease expiration date	Oct. 31, 2020
Lease commenced date	2016-07
Monthly lease payments	$ 25,000
Lease expiration term	51 months
Letters of credit	$ 225,000

Stock-based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			9 Months Ended
	Jan. 02, 2016	Sep. 16, 2015	Sep. 30, 2016	Sep. 30, 2015	Jan. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Proceeds from exercise of stock options			$ 174	$ 100
Unrecognized stock-based compensation expense related to employees' awards			$ 19,003
Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation, weighted-average period			3 years 2 months 23 days
2015 Equity Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares authorized for issuance		2,952,000	7,178,000
Percentage of annual increase in number of shares available for future issuance			4.00%
Additional shares to be issued	1,053,000
Shares available for future grants			1,848,000
Stock options expiration date			10 years
Stock options vesting period			4 years
Stock plan expiration date			2025-06
Weighted-average fair values of options granted			$ 8.18
Exercise of stock options, Shares			162,000
Total intrinsic value of options exercised			$ 1,692
2015 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, percentage of outstanding stock minimum		1.00%
Share-based compensation arrangement by share-based payment award, description			The number of shares reserved for issuance under the 2015 ESPP automatically increases on the first business day of each fiscal year, commencing in 2016
Additional shares to be authorized					0
2015 Employee Stock Purchase Plan [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares authorized for issuance		254,000

Stock-based Compensation - Summary of Stock Option Activity (Detail) - 2014 and 2015 Equity Incentive Plan [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]
Shares Outstanding, Beginning Balance	3,684
Shares, Granted	1,696
Shares, Exercised	(162)
Shares, Cancelled or forfeited	(178)
Shares Outstanding, Ending Balance	5,040	3,684
Shares, Exercisable	1,898
Shares, Vested and expected to vest	5,040
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-average Exercise Price Outstanding, Beginning Balance	$ 5.52
Weighted-average Exercise Price, Granted	12.30
Weighted-average Exercise Price, Exercised	1.08
Weighted-average Exercise Price, Cancelled or forfeited	14.62
Weighted-average Exercise Price, Outstanding, Ending Balance	7.62	$ 5.52
Weighted-average Exercise Price, Exercisable	3.26
Weighted-average Exercise Price, Vested and expected to vest	$ 7.62
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average Remaining Contractual Life (Years) Outstanding	8 years 8 months 12 days	9 years 1 month 6 days
Weighted-average Remaining Contractual Life (Years), Exercisable	8 years 3 months 18 days
Weighted-average Remaining Contractual Life (Years), Vested and expected to vest	8 years 8 months 12 days
Aggregate Intrinsic Value Outstanding	$ 58,033	$ 44,472
Aggregate Intrinsic Value, Exercisable	23,110
Aggregate Intrinsic Value, Vested and expected to vest	$ 58,033

Related Party Transactions - Additional Information (Detail) - USD ($) shares in Thousands		1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 02, 2016	Jan. 31, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2014	Dec. 31, 2015
Related Party Transaction [Line Items]
License revenue from license agreements			$ 65,000	$ 65,000	$ 2,638,000	$ 635,000
Liability related to undelivered material								$ 127,000
Service Agreements [Member]
Related Party Transaction [Line Items]
Amount received for purchase of materials from related party	$ 80,000
The Trustees of the University of Pennsylvania [Member] \| Manufacturing of Reagents for Sale [Member]
Related Party Transaction [Line Items]
Related party transaction expense				40,000		80,000
The Trustees of the University of Pennsylvania [Member] \| Research and Development Services [Member]
Related Party Transaction [Line Items]
Related party transaction expense				2,493,000		5,136,000
Dimension Therapeutics, Inc. [Member]
Related Party Transaction [Line Items]
License revenue from license agreements				1,000,000		2,000,000
Amount received for purchase of materials from related party							$ 200,000
Gain on disposal of the material to related party				5,000		26,000	$ 73,000
Liability related to undelivered material				127,000		127,000
FoxKiser,LLP [Member]
Related Party Transaction [Line Items]
Monthly lease payments		$ 20,000
Rent expense				0		60,000
FoxKiser,LLP [Member] \| Service Agreements [Member]
Related Party Transaction [Line Items]
Related party transaction expense				0		345,000
Related party transaction, description					The amounts outstanding to FoxKiser under the services agreement in excess of 30 days from their due date accrued interest at 1.5 percent per month, compounding monthly.
Accrued interest compounding monthly on amount outstanding					1.50%
FoxKiser,LLP [Member] \| Service Agreements [Member] \| Research and Development Costs [Member]
Related Party Transaction [Line Items]
Related party transaction expense			240,000		$ 720,000
Chief Scientific Advisor [Member]
Related Party Transaction [Line Items]
Related party transaction expenses from transactions with related party advisory fees			$ 62,000	$ 45,000	$ 187,000	$ 135,000
Shares, Granted					212

Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2016	Dec. 31, 2015
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	26,475,000	26,313,000
Common stock, shares outstanding	26,475,000	26,313,000

Property and Equipment Net - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Property Plant And Equipment [Line Items]
Total property and equipment	$ 6,148	$ 618
Accumulated depreciation and amortization	(344)	(80)
Property and equipment, net	5,804	538
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	865	458
Lab Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	756
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	870	105
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	$ 3,657	$ 55

Significant Agreements - Additional Information (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2016 USD ($)
License Agreements [Abstract]
Milestone fee upon submission of preclinical regulatory filings	$ 500
Milestone fee upon commencement of clinical trials in humans	23,650
Milestone fee upon submission of regulatory approval filings	37,000
Milestone fee upon approval of commercial products by regulatory agencies	91,500
Milestone fee upon achievement of specified sales targets for licensed products	$ 92,000

Stock-based Compensation - Schedule of Employees Stock Purchase Plan Valuation Assumptions (Detail)	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	75.00%	67.00%
Expected term (in years)	6 years 2 months 12 days	6 years 1 month 6 days
Risk-free interest rate	1.50%	1.70%
Expected dividend yield	0.00%	0.00%
Non Employee Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	83.00%	71.00%
Expected term (in years)	8 years 1 month 6 days	9 years 8 months 12 days
Risk-free interest rate	1.60%	2.10%
Expected dividend yield	0.00%	0.00%

Net Loss Per Share - Schedules for Computation of Diluted Weighted-Average Shares Outstanding (Detail) - shares shares in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	5,052	3,464
ESPP Shares [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	12
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	5,040	3,464

Nature of Business	9 Months Ended
Nature of Business	Sep. 30, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of Business	1. Nature of Business REGENXBIO Inc. (the Company) was formed on July 16, 2008 in the state of Delaware as ReGenX, LLC, and on December 22, 2009, changed its name to ReGenX Biosciences, LLC. On September 16, 2014, the Company converted from a limited liability company (LLC) to a C-corporation, and changed its name to REGENXBIO Inc. The Company is a leading biotechnology company focused on the development, commercialization and licensing of recombinant adeno-associated virus (AAV) gene therapy. The Company’s proprietary 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, in the development of product candidates for a variety of diseases with unmet needs. Initial Public Offering On September 22, 2015, the Company completed its initial public offering (IPO) whereby the Company sold 7,245 shares of common stock (inclusive of 945 shares of common stock sold by the Company pursuant to the full exercise of an option to purchase additional shares granted to the underwriters in connection with the offering) at a price of $22.00 per share. The shares began trading on The Nasdaq Global Select Market on September 17, 2015. The aggregate net proceeds received by the Company from the offering were $145,184, net of underwriting discounts and commissions and offering expenses payable by the Company. Upon the closing of the IPO, all outstanding shares of convertible preferred stock converted into 16,298 shares of common stock. Liquidity and Risks As of September 30, 2016, the Company had generated an accumulated deficit of $94,979 since inception. As the Company continues to incur losses, transition to 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 profitability, and unless and until it does, the Company will continue to need to raise additional capital. As of September 30, 2016, the Company had cash, cash equivalents and marketable securities of $184,857, which management believes is sufficient to fund operations for at least the next 12 months. 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 preclinical manufacturing to commercial production of products.

Marketable Securities - Summary of Company Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 155,896	$ 162,970
Unrealized Gains	887	40
Unrealized Losses	(34)	(759)
Fair Value	156,749	162,251
Common Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	300	3
Unrealized Gains	595	36
Fair Value	895	39
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	155,596	157,977
Unrealized Gains	292	4
Unrealized Losses	(34)	(759)
Fair Value	$ 155,854	157,222
Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		4,990
Fair Value		$ 4,990

Fair Value of Financial Instruments - Schedule of Fair Value of Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	$ 156,749	$ 162,251
Assets fair value disclosure	184,857	216,355
Common Equity Securities [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	895	39
Corporate Bonds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	155,854	157,222
Commercial Paper [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities		4,990
Money Market Mutual Funds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Money market mutual funds (cash equivalents)	28,108	54,104
Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets fair value disclosure	895	39
Quoted Prices in Active Markets (Level 1) [Member] \| Common Equity Securities [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	895	39
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets fair value disclosure	183,962	216,316
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	155,854	157,222
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		4,990
Significant Other Observable Inputs (Level 2) [Member] \| Money Market Mutual Funds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Money market mutual funds (cash equivalents)	$ 28,108	$ 54,104

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases (Detail) $ in Thousands	Sep. 30, 2016 USD ($)
Operating Leases Future Minimum Payments Due [Abstract]
2016 (remainder of year)	$ 387
2017	1,589
2018	1,637
2019	1,685
2020	1,611
Thereafter	1,614
Total minimum lease payments	$ 8,523

Supplemental Disclosures - Schedules of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Payables And Accruals [Abstract]
Accrued external research and development expenses	$ 3,721	$ 835
Accrued personnel costs	3,339	1,371
Accrued purchases of property and equipment	1,055
Accrued external general and administrative expenses	449	559
Accrued sublicense fees and royalties on reagent sales		260
Other accrued expenses and current liabilities	442	173
Accrued expenses and other current liabilities	$ 9,006	$ 3,198