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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2017	May 05, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2017
Document Fiscal Year Focus	2017
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		30,821,355

Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Current assets
Cash and cash equivalents	$ 82,045	$ 24,840
Marketable securities	63,764	64,714
Accounts receivable	228	1,032
Prepaid expenses	1,843	1,775
Other current assets	922	1,010
Total current assets	148,802	93,371
Marketable securities	63,742	69,412
Property and equipment, net	11,061	9,324
Restricted cash	225	225
Other assets	297	400
Total assets	224,127	172,732
Current liabilities
Accounts payable	4,129	1,543
Accrued expenses and other current liabilities	5,829	8,126
Total current liabilities	9,958	9,669
Deferred rent, net of current portion	1,271	1,326
Total liabilities	11,229	10,995
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, 2017 and December 31, 2016
Common stock; $0.0001 par value; 100,000 shares authorized at March 31, 2017 and December 31, 2016; 30,244 and 26,477 shares issued and outstanding at March 31, 2017 and December 31, 2016, respectively	3	3
Additional paid-in capital	350,047	276,354
Accumulated other comprehensive loss	(572)	(33)
Accumulated deficit	(136,580)	(114,587)
Total stockholders’ equity	212,898	161,737
Total liabilities and stockholders’ equity	$ 224,127	$ 172,732

Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2017	Dec. 31, 2016
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	30,244,000	26,477,000
Common stock, shares outstanding	30,244,000	26,477,000

Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Revenues
License revenue	$ 455	$ 328
Reagent sales		59
Grant revenue		6
Total revenues	455	393
Costs of revenues
Licensing costs	91	66
Costs of reagent sales		30
Research and development	16,619	6,183
General and administrative	6,622	5,479
Other operating expenses (income)	45	(114)
Total operating expenses	23,377	11,644
Loss from operations	(22,922)	(11,251)
Other Income
Investment income	929	483
Total other income	929	483
Net loss	(21,993)	(10,768)
Other Comprehensive Income (Loss)
Unrealized gain (loss) on available-for-sale securities, net of reclassifications of $480 for the three months ended March 31, 2017	(539)	994
Total other comprehensive income (loss)	(539)	994
Comprehensive loss	(22,532)	(9,774)
Net loss applicable to common stockholders	$ (21,993)	$ (10,768)
Basic and diluted net loss per common share	$ (0.82)	$ (0.41)
Weighted-average basic and diluted common shares	26,673	26,327

Statements of Operations and Comprehensive Loss (Parenthetical) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($)
Income Statement [Abstract]
Unrealized gain (loss) on available-for-sale securities, net of reclassifications	$ 480

Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
Statements of Cash Flows - USD ($) $ in Thousands	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities
Net loss	$ (21,993)	$ (10,768)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	2,591	1,546
Net amortization of premiums and accretion of discounts on marketable debt securities	469	471
Depreciation and amortization	554	48
Realized gains on sales of marketable securities	(480)
Other non-cash adjustments	41	(13)
Changes in operating assets and liabilities
Accounts receivable	803	113
Prepaid expenses	(68)	(169)
Other current assets	88	(449)
Other assets	10	(55)
Accounts payable	2,468	(182)
Accrued expenses and other current liabilities	(1,920)	1,210
Advance payments		(92)
Deferred rent	(44)	453
Net cash used in operating activities	(17,481)	(7,887)
Cash flows from investing activities
Purchases of marketable securities	(5,188)	(20,429)
Maturities of marketable securities	10,500	10,106
Sales of marketable securities	780
Purchases of property and equipment	(2,929)	(416)
Net cash provided by (used in) investing activities	3,163	(10,739)
Cash flows from financing activities
Proceeds from exercise of stock options	161	21
Proceeds from issuance of common stock under employee stock purchase plan	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	71,523	21
Net increase (decrease) in cash and cash equivalents	57,205	(18,605)
Cash and cash equivalents
Beginning of period	24,840	54,116
End of period	82,045	35,511
Supplemental disclosures of non-cash investing and financing activities
Purchases of property and equipment in accounts payable and accrued expenses		$ 422
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, 2017
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, 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. Follow-on Public Offering On March 27, 2017, the Company completed a follow-on public offering whereby the Company sold 3,700,000 shares of common stock at a price of $20.50 per share. The aggregate net proceeds received by the Company from the offering were $70.8 million, net of underwriting discounts and commissions and offering expenses payable by the Company. In connection with the offering, the Company granted the underwriters an option to purchase up to 555,000 additional shares of common stock at the public offering price. On April 26, 2017, the option was exercised in full, resulting in an additional $10.7 million in net proceeds received by the Company, net of underwriting discounts and commissions. Liquidity and Risks As of March 31, 2017, the Company had generated an accumulated deficit of $136.6 million 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 March 31, 2017, the Company had cash, cash equivalents and marketable securities of $209.6 million, which management believes is sufficient to fund operations for at least the next 12 months from the date these 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 commercial production of products.

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2017

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, 2016 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016, filed with the Securities and Exchange Commission (SEC) on March 7, 2017. Certain information and footnote disclosures required by GAAP which are normally included in the Company’s annual financial statements have been 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 March 31, 2017, and the results of its operations and its cash flows for the interim periods ended March 31, 2017 and 2016.

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, 2016, 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.

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

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, outstanding stock options, outstanding restricted stock units 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 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. Additionally, the standard allows companies to elect not to estimate forfeitures of share-based awards for purposes of recognizing stock-based compensation expense, and account for forfeitures as they occur. The standard is effective for annual and interim periods beginning after December 15, 2016, with early adoption permitted upon issuance. The Company has adopted this standard effective January 1, 2017, and upon adoption, has elected not to estimate forfeitures of share-based awards, which requires retrospective application in the financial statements. The Company has estimated a forfeiture rate of zero on all share-based awards granted since its inception. Additionally, the adoption of this standard had no material impact on the Company’s tax provision or classification of the Company’s share-based awards. Accordingly, the application of this standard had no material impact on the Company’s financial position or results of operations.

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. 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 2015-14, Revenue from Contracts with Customers (Topic 606), which deferred the effective date of the guidance under ASU 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. In December 2016, the FASB issued ASU 2016-20, Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers, which clarifies various additional narrow aspects of 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 has elected not to early adopt these standards. The Company is evaluating the application of these standards, but has not yet determined the potential effects they may have on the Company’s financial statements.

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 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 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 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.

Marketable Securities

3 Months Ended

Mar. 31, 2017

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 securities (in thousands):

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
March 31, 2017
Corporate bonds	$	127,643	$	35	$	(172	)	$	127,506
	$	127,643	$	35	$	(172	)	$	127,506

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
December 31, 2016
Corporate bonds	$	133,424	$	82	$	(298	)	$	133,208
Common equity securities		300		618		—			918
	$	133,724	$	700	$	(298	)	$	134,126

Common equity securities reported as of December 31, 2016 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. As of March 31, 2017, the Company had sold all of its shares of Audentes common stock.

As of March 31, 2017 and December 31, 2016, 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, 2017 and December 31, 2016, 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. For 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 available-for-sale 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 statements of operations and comprehensive loss.

The Company did not hold any securities in an unrealized loss position for more than 12 months as of March 31, 2017 or December 31, 2016. The aggregate fair value of securities held by the Company in an unrealized loss position for less than 12 months as of March 31, 2017 and December 31, 2016 was $89.9 million and $102.5 million, respectively. As of March 31, 2017, securities held by the Company which were in an unrealized loss position consisted of 32 investment grade corporate bond 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, 2017 or December 31, 2016.

- Definition

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

+ References + Details

Fair Value of Financial Instruments

3 Months Ended

Mar. 31, 2017

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, 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 (in thousands):

	Quoted		Significant
	prices		other		Significant
	in active		observable		unobservable
	markets		inputs		inputs
	(Level 1)		(Level 2)		(Level 3)		Total
March 31, 2017
Money market mutual funds (cash equivalents)	$	—	$	82,045	$	—	$	82,045
Corporate bonds (marketable securities)		—		127,506		—		127,506
	$	—	$	209,551	$	—	$	209,551

	Quoted		Significant
	prices		other		Significant
	in active		observable		unobservable
	markets		inputs		inputs
	(Level 1)		(Level 2)		(Level 3)		Total
December 31, 2016
Money market mutual funds (cash equivalents)	$	—	$	24,840	$	—	$	24,840
Corporate bonds (marketable securities)		—		133,208		—		133,208
Common equity securities (marketable securities)		918		—		—		918
	$	918	$	158,048	$	—	$	158,966

There were no transfers of financial instruments between levels of the fair value hierarchy during the three months ended March 31, 2017.

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.

- 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, 2017

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, 2017			December 31, 2016
Computer equipment and software	$	1,167		$	1,038
Lab equipment		4,866			2,780
Furniture and fixtures		1,251			1,213
Leasehold improvements		4,955			4,917
Total property and equipment		12,239			9,948
Accumulated depreciation and amortization		(1,178	)		(624	)
Property and equipment, net	$	11,061		$	9,324

Commitments and Contingencies

3 Months Ended

Mar. 31, 2017

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. Monthly payments under the lease began in October 2015 and escalate annually in accordance with the lease agreement.

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. Monthly payments under the lease escalate annually in accordance with the lease agreement. The Company received a $0.3 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 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. Monthly payments under the lease began in September 2016 and escalate annually in accordance with the lease agreement. The Company received a $0.7 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 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, 2017, 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, 2017, future minimum lease payments under non-cancelable operating leases are as follows (in thousands):

	Operating
	Leases
2017 (remainder of year)	$	1,199
2018		1,638
2019		1,686
2020		1,605
2021		665
Thereafter		952
Total minimum lease payments	$	7,745

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 (in thousands):

	Three Months Ended March 31,
	2017		2016
Sublicense fees	$	46	$	33
Royalties on sales of reagents		—		2
Maintenance of licensed patents		84		29
	$	130	$	64

As of March 31, 2017 and December 31, 2016, the Company had accrued $0.1 million and $0.2 million, 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.

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 $1.5 million upon the achievement of various milestones. As of March 31, 2017, 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 (in thousands):

	Three Months Ended March 31,
	2017		2016
Sublicense fees	$	46	$	33
Royalties on sales of reagents		—		1
Maintenance of licensed patents		145		92
	$	191	$	126

As of March 31, 2017 and December 31, 2016, the Company had accrued $0.2 million and $0.4 million, 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.

Regents of the University of Minnesota. In November 2014, the Company entered into a license agreement, as amended, 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.

As of March 31, 2017, 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 license agreement. As of March 31, 2017 and December 31, 2016, the Company had accrued less than $0.1 million in patent maintenance expenses and up-front fees 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 March 31, 2017 and December 31, 2016, 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

3 Months Ended

Mar. 31, 2017

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 Platform. 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 March 31, 2017, 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 $0.5 million upon the submission of preclinical regulatory filings, $24.7 million upon the commencement of various stages of clinical trials, $40.0 million upon the submission of regulatory approval filings, $97.5 million upon the approval of commercial products by regulatory agencies and $92.0 million upon the achievement of specified sales targets for licensed products.

License revenue consists of the following (in thousands):

	Three Months Ended March 31,
	2017		2016
Maintenance fees for commercial licenses	$	250	$	250
Research and other license revenue		205		78
	$	455	$	328

Stock-based Compensation

3 Months Ended

Mar. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-based Compensation

8. Stock-based Compensation

In January 2017, an additional 1,059,065 shares became available for issuance under the 2015 Equity Incentive Plan (2015 Plan). As of March 31, 2017, the total number of shares of common stock authorized for issuance under the 2015 Plan and 2014 Stock Plan (2014 Plan) was 8,236,603, of which 2,137,390 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,
	2017		2016
Stock options	$	2,439	$	1,546
Restricted stock units		69		—
Employee stock purchase plan		83		—
	$	2,591	$	1,546

As of March 31, 2017, the Company had $26.2 million of unrecognized stock-based compensation expense related to stock options, restricted stock units and the 2015 Employee Stock Purchase Plan (2015 ESPP), which is expected to be recognized over a weighted-average period of 3.2 years.

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

	Three Months Ended March 31,
	2017		2016
Research and development	$	1,238	$	392
General and administrative		1,353		1,154
	$	2,591	$	1,546

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, 2016		5,118		$	7.77		8.5	$	57,400
Granted		758		$	19.28
Exercised		(46	)	$	3.54
Cancelled or forfeited		(106	)	$	13.52
Outstanding at March 31, 2017		5,724		$	9.22		8.5	$	59,552
Exercisable at March 31, 2017		2,612		$	5.15		7.9	$	37,632
Vested and expected to vest at March 31, 2017		5,724		$	9.22		8.5	$	59,552

(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, 2017 was $13.18. During the three months ended March 31, 2017, the total number of stock options exercised was 45,570, resulting in total proceeds of $0.2 million. The total intrinsic value of options exercised during the three months ended March 31, 2017 was $0.7 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, 2016		40	$	20.90
Granted		—	$	—
Vested		—	$	—
Forfeited		—	$	—
Unvested balance at March 31, 2017		40	$	20.90

Employee Stock Purchase Plan

As of March 31, 2017, the total number of shares of common stock authorized for issuance under the 2015 ESPP was 254,000, of which 232,294 remain available for future issuance. During the three months ended March 31, 2017, 21,706 shares of common stock were 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	3 Months Ended
Related Party Transactions	Mar. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	9. Related Party Transactions FOXKISER LLP Effective January 2016, the Company entered into a Professional Services Agreement 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, pursuant to which the Company incurs a fixed monthly fee in consideration for certain strategic planning, development and regulatory services provided by FOXKISER. The agreement expired in December 2016, and effective January 2017 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 sixty days’ prior written notice to the other party. Costs incurred under the agreements with FOXKISER for the three months ended March 31, 2017 and 2016 were $0.4 million and $0.2 million, respectively, which have been recorded as research and development expenses in the statements of operations and comprehensive loss. Special Scientific 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. The advisor 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 and 2016, the Company incurred advisory fees of $0.1 million and $0.1 million, respectively, under the agreements. Additionally, the Company has granted options to purchase 211,600 shares of common stock to the Scientific Founder and Special Advisor as compensation for 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 Loss Per Share

3 Months Ended

Mar. 31, 2017

Earnings Per Share [Abstract]

Net Loss Per Share

10. Net Loss Per Share

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

	Three Months Ended March 31,
	2017		2016
Stock options issued and outstanding		5,724		4,718
Unvested restricted stock units outstanding		40		—
Employee stock purchase plan		16		—
		5,780		4,718

Amounts in the table above reflect the common stock equivalents of the noted instruments.

Supplemental Disclosures

3 Months Ended

Mar. 31, 2017

Payables And Accruals [Abstract]

Supplemental Disclosures

11. Supplemental Disclosures

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

	March 31, 2017		December 31, 2016
Accrued personnel costs	$	2,965	$	3,338
Accrued external research and development expenses		1,586		3,005
Accrued external general and administrative expenses		819		672
Accrued purchases of property and equipment		129		844
Other accrued expenses and current liabilities		330		267
	$	5,829	$	8,126

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2017

Accounting Policies [Abstract]

Basis of Presentation and Unaudited Interim Financial Information

Use of Estimates

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)

3 Months Ended

Mar. 31, 2017

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 securities (in thousands):

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
March 31, 2017
Corporate bonds	$	127,643	$	35	$	(172	)	$	127,506
	$	127,643	$	35	$	(172	)	$	127,506

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
December 31, 2016
Corporate bonds	$	133,424	$	82	$	(298	)	$	133,208
Common equity securities		300		618		—			918
	$	133,724	$	700	$	(298	)	$	134,126

- 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, 2017

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, 2017
Money market mutual funds (cash equivalents)	$	—	$	82,045	$	—	$	82,045
Corporate bonds (marketable securities)		—		127,506		—		127,506
	$	—	$	209,551	$	—	$	209,551

	Quoted		Significant
	prices		other		Significant
	in active		observable		unobservable
	markets		inputs		inputs
	(Level 1)		(Level 2)		(Level 3)		Total
December 31, 2016
Money market mutual funds (cash equivalents)	$	—	$	24,840	$	—	$	24,840
Corporate bonds (marketable securities)		—		133,208		—		133,208
Common equity securities (marketable securities)		918		—		—		918
	$	918	$	158,048	$	—	$	158,966

Property and Equipment, Net (Tables)

3 Months Ended

Mar. 31, 2017

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment

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

	March 31, 2017			December 31, 2016
Computer equipment and software	$	1,167		$	1,038
Lab equipment		4,866			2,780
Furniture and fixtures		1,251			1,213
Leasehold improvements		4,955			4,917
Total property and equipment		12,239			9,948
Accumulated depreciation and amortization		(1,178	)		(624	)
Property and equipment, net	$	11,061		$	9,324

Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2017

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

As of March 31, 2017, future minimum lease payments under non-cancelable operating leases are as follows (in thousands):

	Operating
	Leases
2017 (remainder of year)	$	1,199
2018		1,638
2019		1,686
2020		1,605
2021		665
Thereafter		952
Total minimum lease payments	$	7,745

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,
	2017		2016
Sublicense fees	$	46	$	33
Royalties on sales of reagents		—		2
Maintenance of licensed patents		84		29
	$	130	$	64

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,
	2017		2016
Sublicense fees	$	46	$	33
Royalties on sales of reagents		—		1
Maintenance of licensed patents		145		92
	$	191	$	126

Significant Agreements (Tables)

3 Months Ended

Mar. 31, 2017

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Schedule of License Revenue

License revenue consists of the following (in thousands):

	Three Months Ended March 31,
	2017		2016
Maintenance fees for commercial licenses	$	250	$	250
Research and other license revenue		205		78
	$	455	$	328

Stock-based Compensation (Tables)

3 Months Ended

Mar. 31, 2017

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,
	2017		2016
Stock options	$	2,439	$	1,546
Restricted stock units		69		—
Employee stock purchase plan		83		—
	$	2,591	$	1,546

Stock-Based Compensation Expense

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

	Three Months Ended March 31,
	2017		2016
Research and development	$	1,238	$	392
General and administrative		1,353		1,154
	$	2,591	$	1,546

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, 2016		40	$	20.90
Granted		—	$	—
Vested		—	$	—
Forfeited		—	$	—
Unvested balance at March 31, 2017		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, 2016		5,118		$	7.77		8.5	$	57,400
Granted		758		$	19.28
Exercised		(46	)	$	3.54
Cancelled or forfeited		(106	)	$	13.52
Outstanding at March 31, 2017		5,724		$	9.22		8.5	$	59,552
Exercisable at March 31, 2017		2,612		$	5.15		7.9	$	37,632
Vested and expected to vest at March 31, 2017		5,724		$	9.22		8.5	$	59,552

(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 Loss Per Share (Tables)

3 Months Ended

Mar. 31, 2017

Earnings Per Share [Abstract]

Schedules for Computation of Diluted Weighted-Average Shares Outstanding

	Three Months Ended March 31,
	2017		2016
Stock options issued and outstanding		5,724		4,718
Unvested restricted stock units outstanding		40		—
Employee stock purchase plan		16		—
		5,780		4,718

Supplemental Disclosures (Tables)

3 Months Ended

Mar. 31, 2017

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, 2017		December 31, 2016
Accrued personnel costs	$	2,965	$	3,338
Accrued external research and development expenses		1,586		3,005
Accrued external general and administrative expenses		819		672
Accrued purchases of property and equipment		129		844
Other accrued expenses and current liabilities		330		267
	$	5,829	$	8,126

Nature of Business - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended
	Apr. 26, 2017	Mar. 27, 2017	Mar. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Net proceeds from issuance of common stock			$ 71,299
Accumulated deficit			(136,580)	$ (114,587)
Cash, cash equivalents and marketable securities			$ 209,600
Follow-on Public Offering [Member]
Class of Stock [Line Items]
Number of shares issued to during period		3,700,000
Stock issuance price per share		$ 20.50
Follow-on Public Offering [Member] \| Common Stock [Member]
Class of Stock [Line Items]
Net proceeds from issuance of common stock		$ 70,800
Overallotment Option [Member] \| Common Stock [Member] \| Subsequent Event [Member]
Class of Stock [Line Items]
Net proceeds from issuance of common stock	$ 10,700
Overallotment Option [Member] \| Maximum [Member] \| Common Stock [Member]
Class of Stock [Line Items]
Number of shares issued to during period		555,000

Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
Estimated forfeiture rate on share-based awards granted	0.00%

Marketable Securities - Summary of Company Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 127,643	$ 133,724
Unrealized Gains	35	700
Unrealized Losses	(172)	(298)
Fair Value	127,506	134,126
Common Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		300
Unrealized Gains		618
Fair Value		918
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	127,643	133,424
Unrealized Gains	35	82
Unrealized Losses	(172)	(298)
Fair Value	$ 127,506	$ 133,208

Marketable Securities - Additional Information (Detail)	3 Months Ended	12 Months Ended
Marketable Securities - Additional Information (Detail)	Mar. 31, 2017 USD ($) Bond	Dec. 31, 2016 USD ($)
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	(100,000)
Unrealized losses available-for-sale securities, income tax expense	0
Realized gains available for sale securities	480,000
Aggregate fair value of securities in an unrealized loss position for less than twelve months	89,900,000	102,500,000
Other-than-temporary impaired	$ 0	$ 0
Number of investment grade corporate bonds \| Bond	32
Dimension Therapeutics, Inc. [Member] \| Common Equity Securities [Member] \| Investment Income [Member]
Schedule Of Available For Sale Securities [Line Items]
Realized gains available for sale securities	$ 500,000

Fair Value of Financial Instruments - Schedule of Fair Value of Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	$ 127,506	$ 134,126
Assets fair value disclosure	209,551	158,966
Common Equity Securities [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities		918
Corporate Bonds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	127,506	133,208
Money Market Mutual Funds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Money market mutual funds (cash equivalents)	82,045	24,840
Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets fair value disclosure		918
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		918
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets fair value disclosure	209,551	158,048
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	127,506	133,208
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)	$ 82,045	$ 24,840

Fair Value of Financial Instruments - Additional Information (Detail)	Mar. 31, 2017 USD ($)
Fair Value Disclosures [Abstract]
Fair value hierarchy level 1 to level 2 transfers amount	$ 0
Fair value hierarchy level 2 to level 1 transfers amount	$ 0

Property and Equipment Net - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Total property and equipment	$ 12,239	$ 9,948
Accumulated depreciation and amortization	(1,178)	(624)
Property and equipment, net	11,061	9,324
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,167	1,038
Lab Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	4,866	2,780
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,251	1,213
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	$ 4,955	$ 4,917

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended							3 Months Ended	12 Months Ended
	Nov. 30, 2016 USD ($)	May 31, 2016 USD ($)	Jan. 31, 2016 USD ($)	Sep. 30, 2015 USD ($)	Mar. 31, 2015	Nov. 30, 2014 USD ($)	Mar. 31, 2009 USD ($)	Mar. 31, 2017 USD ($) Milestone	Dec. 31, 2016 USD ($)
Other Commitments [Line Items]
Tenant improvement allowance received				$ 300,000
Restricted cash as collateral with financial institution								$ 225,000	$ 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								$ 100,000	200,000
GlaxoSmithKline LLC [Member]
Other Commitments [Line Items]
Percentage of ownership interest issued as consideration for license agreement							19.90%
Accrued expenses								$ 200,000	400,000
Number of milestone \| Milestone								0
Milestone payment obligation							$ 1,500,000	$ 0
Regents of the University of Minnesota [Member]
Other Commitments [Line Items]
Number of milestone \| Milestone								0
Milestone payments								$ 0
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								$ 100,000	$ 100,000
Regents of the University of Minnesota [Member] \| Annual License Maintenance Fee [Member] \| Maximum [Member]
Other Commitments [Line Items]
Contractual obligation						100,000
Regents of the University of Minnesota [Member] \| Royalties And Sublicense Fees [Member] \| Maximum [Member]
Other Commitments [Line Items]
Contractual obligation						$ 100,000
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
Lease term								Extend the term of the lease for its existing space to October 2020.
Tenant improvement allowance received			$ 700,000
Monthly lease payments commencement period			2016-09
New York, New York [Member]
Other Commitments [Line Items]
Lease expiration date		Oct. 31, 2020
Lease commenced date		2016-07
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, 2017 USD ($)
Operating Leases Future Minimum Payments Due [Abstract]
2017 (remainder of year)	$ 1,199
2018	1,638
2019	1,686
2020	1,605
2021	665
Thereafter	952
Total minimum lease payments	$ 7,745

Commitments and Contingencies - Summary of Expenses Incurred by Company (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
GlaxoSmithKline LLC [Member]
Other Commitments [Line Items]
Total related party transaction expense	$ 191	$ 126
GlaxoSmithKline LLC [Member] \| Sublicense Fees [Member]
Other Commitments [Line Items]
Total related party transaction expense	46	33
GlaxoSmithKline LLC [Member] \| Royalties on Sales of Reagents [Member]
Other Commitments [Line Items]
Total related party transaction expense		1
GlaxoSmithKline LLC [Member] \| Maintenance of Licensed Patents [Member]
Other Commitments [Line Items]
Total related party transaction expense	145	92
License Fees [Member] \| The Trustees of the University of Pennsylvania [Member]
Other Commitments [Line Items]
Total related party transaction expense	130	64
License Fees [Member] \| The Trustees of the University of Pennsylvania [Member] \| Sublicense Fees [Member]
Other Commitments [Line Items]
Total related party transaction expense	46	33
License Fees [Member] \| The Trustees of the University of Pennsylvania [Member] \| Royalties on Sales of Reagents [Member]
Other Commitments [Line Items]
Total related party transaction expense		2
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	$ 84	$ 29

Significant Agreements - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($)
License Agreements [Abstract]
Milestone fee upon submission of preclinical regulatory filings	$ 0.5
Milestone fee upon commencement of clinical trials in humans	24.7
Milestone fee upon submission of regulatory approval filings	40.0
Milestone fee upon approval of commercial products by regulatory agencies	97.5
Milestone fee upon achievement of specified sales targets for licensed products	$ 92.0

Significant Agreements - Schedule of License Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
License Agreement Revenue Recognition [Abstract]
Maintenance fees for commercial licenses	$ 250	$ 250
Research and other license revenue	205	78
Licenses revenue including amounts from related parties	$ 455	$ 328

Stock-based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
	Jan. 31, 2017	Mar. 31, 2017	Mar. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense		$ 26,200
Proceeds from exercise of stock options		161	$ 21
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense		$ 26,200
Unrecognized stock-based compensation, weighted-average period		3 years 2 months 12 days
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense		$ 26,200
Unrecognized stock-based compensation, weighted-average period		3 years 2 months 12 days
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 12 days
2015 Equity Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional shares to be issued	1,059,065
Common stock shares authorized for issuance		8,236,603
Shares available for future grants		2,137,390
Weighted-average fair values of options granted		$ 13.18
Exercise of stock options, Shares		45,570
Total intrinsic value of options exercised		$ 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		232,294
Common stock shares issued to participants		21,706

Stock-based Compensation - Stock-Based Compensation Expense by Award Type (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 2,591	$ 1,546
Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	2,439	$ 1,546
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	69
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 83

Stock-based Compensation - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 2,591	$ 1,546
Research and Development Costs [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	1,238	392
General and Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 1,353	$ 1,154

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, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]
Shares Outstanding, Beginning Balance	5,118
Shares, Granted	758
Shares, Exercised	(46)
Shares, Cancelled or forfeited	(106)
Shares Outstanding, Ending Balance	5,724	5,118
Shares, Exercisable	2,612
Shares, Vested and expected to vest	5,724
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-average Exercise Price Outstanding, Beginning Balance	$ 7.77
Weighted-average Exercise Price, Granted	19.28
Weighted-average Exercise Price, Exercised	3.54
Weighted-average Exercise Price, Cancelled or forfeited	13.52
Weighted-average Exercise Price, Outstanding, Ending Balance	9.22	$ 7.77
Weighted-average Exercise Price, Exercisable	5.15
Weighted-average Exercise Price, Vested and expected to vest	$ 9.22
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average Remaining Contractual Life (Years) Outstanding	8 years 6 months	8 years 6 months
Weighted-average Remaining Contractual Life (Years), Exercisable	7 years 10 months 24 days
Weighted-average Remaining Contractual Life (Years), Vested and expected to vest	8 years 6 months
Aggregate Intrinsic Value Outstanding	$ 59,552	$ 57,400
Aggregate Intrinsic Value, Exercisable	37,632
Aggregate Intrinsic Value, Vested and expected to vest	$ 59,552

Stock-based Compensation - Summary of Unvested RSUs Activity Under 2015 Plan (Detail) - Restricted Stock Units [Member] shares in Thousands	Mar. 31, 2017 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Unvested, Beginning Balance \| shares	40
Shares Unvested, Ending Balance \| shares	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
	Mar. 31, 2017	Mar. 31, 2016
FOXKISER,LLP [Member] \| Service Agreements [Member] \| Research and Development Costs [Member]
Related Party Transaction [Line Items]
Related party transaction expense	$ 0.4	$ 0.2
Special Scientific Advisor [Member]
Related Party Transaction [Line Items]
Related party transaction expenses from transactions with related party advisory fees	$ 0.1	$ 0.1
Shares, Granted	211,600

Net Loss Per Share - Schedules for Computation of Diluted Weighted-Average Shares Outstanding (Detail) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	5,780	4,718
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	5,724	4,718
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, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accrued personnel costs	$ 2,965	$ 3,338
Accrued external research and development expenses	1,586	3,005
Accrued external general and administrative expenses	819	672
Accrued purchases of property and equipment	129	844
Other accrued expenses and current liabilities	330	267
Accrued expenses and other current liabilities	$ 5,829	$ 8,126