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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2020	May 01, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2020
Document Fiscal Year Focus	2020
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	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity File Number	001-37553
Entity Tax Identification Number	47-1851754
Entity Address, Address Line One	9600 Blackwell Road
Entity Address, Address Line Two	Suite 210
Entity Address, City or Town	Rockville
Entity Address, State or Province	MD
Entity Address, Postal Zip Code	20850
City Area Code	240
Local Phone Number	552-8181
Entity Common Stock, Shares Outstanding		37,241,119
Document Transition Report	false
Document Quarterly Report	true
Entity Interactive Data Current	Yes
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Entity Incorporation, State or Country Code	DE

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets
Cash and cash equivalents	$ 69,411	$ 69,514
Marketable securities	209,846	226,696
Accounts receivable	44,522	38,148
Prepaid expenses	7,422	6,475
Other current assets	7,032	4,199
Total current assets	338,233	345,032
Marketable securities	77,361	103,785
Accounts receivable	4,373	4,155
Property and equipment, net	30,414	28,973
Operating lease right-of-use assets	9,375	10,078
Restricted cash	1,330	1,330
Other assets	2,850	4,555
Total assets	463,936	497,908
Current liabilities
Accounts payable	9,008	6,409
Accrued expenses and other current liabilities	20,327	24,846
Operating lease liabilities	2,454	2,421
Total current liabilities	31,789	33,676
Deferred revenue	3,333	3,333
Operating lease liabilities	7,990	8,874
Other liabilities	672	1,828
Total liabilities	43,784	47,711
Stockholders’ equity
Preferred stock; $0.0001 par value; 10,000 shares authorized, and no shares issued and outstanding at March 31, 2020 and December 31, 2019
Common stock; $0.0001 par value; 100,000 shares authorized at March 31, 2020 and December 31, 2019; 37,190 and 36,992 shares issued and outstanding at March 31, 2020 and December 31, 2019, respectively	4	4
Additional paid-in capital	638,588	627,810
Accumulated other comprehensive income (loss)	(580)	205
Accumulated deficit	(217,860)	(177,822)
Total stockholders’ equity	420,152	450,197
Total liabilities and stockholders’ equity	$ 463,936	$ 497,908

Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2020	Dec. 31, 2019
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	37,190,000	36,992,000
Common stock, shares outstanding	37,190,000	36,992,000

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenues
Revenues	$ 17,644	$ 884
Types of Revenue[Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating Expenses
Cost of revenues	$ 3,409	$ 29
Research and development	37,035	25,203
General and administrative	14,833	11,558
Other operating expenses	67
Total operating expenses	55,344	36,790
Loss from operations	(37,700)	(35,906)
Other Income (Loss)
Interest income from licensing	848	613
Investment income (loss)	(3,186)	2,995
Total other income (loss)	(2,338)	3,608
Loss before income taxes	(40,038)	(32,298)
Income Tax Benefit		70
Net loss	(40,038)	(32,228)
Other Comprehensive Income (Loss)
Unrealized gain (loss) on available-for-sale securities, net	(785)	621
Total other comprehensive income (loss)	(785)	621
Comprehensive loss	$ (40,823)	$ (31,607)
Basic and diluted net loss per share	$ (1.08)	$ (0.89)
Weighted-average basic and diluted common shares outstanding	37,104	36,366

Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional paid in capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Balances at Dec. 31, 2018	$ 508,848	$ 4	$ 592,580	$ (720)	$ (83,016)
Balances (Shares) at Dec. 31, 2018		36,120
Adoption of ASU \| ASU 2016-02 (Topic 842) [Member]	(33)				(33)
Adoption of ASU \| ASU 2018-02 [Member]				40	(40)
Exercise of stock options	3,762		3,762
Exercise of stock options, Shares		481
Issuance of common stock under employee stock purchase plan	365		365
Issuance of common stock under employee stock purchase plan, shares		10
Stock-based compensation expense	5,718		5,718
Unrealized gain (loss) on available-for-sale securities, net	621			621
Net income (loss)	(32,228)				(32,228)
Balances at Mar. 31, 2019	487,053	$ 4	602,425	(59)	(115,317)
Balances (Shares) at Mar. 31, 2019		36,611
Balances at Dec. 31, 2019	450,197	$ 4	627,810	205	(177,822)
Balances (Shares) at Dec. 31, 2019		36,992
Exercise of stock options	2,154		2,154
Exercise of stock options, Shares		181
Issuance of common stock under employee stock purchase plan	607		607
Issuance of common stock under employee stock purchase plan, shares		17
Stock-based compensation expense	8,017		8,017
Unrealized gain (loss) on available-for-sale securities, net	(785)			(785)
Net income (loss)	(40,038)				(40,038)
Balances at Mar. 31, 2020	$ 420,152	$ 4	$ 638,588	$ (580)	$ (217,860)
Balances (Shares) at Mar. 31, 2020		37,190

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities
Net loss	$ (40,038)	$ (32,228)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	8,017	5,718
Net amortization of premiums and accretion of discounts on marketable debt securities	39	(368)
Depreciation and amortization	1,994	1,614
Net realized gains on sales and maturities of marketable securities	(7,085)
Imputed interest income from licensing	(848)	(613)
Unrealized losses on marketable equity securities	12,196
Other non-cash adjustments	25	327
Changes in operating assets and liabilities
Accounts receivable	(5,744)	591
Prepaid expenses	(947)	(790)
Other current assets	(2,833)	(295)
Operating lease right-of-use assets	703	573
Other assets	1,705	26
Accounts payable	3,063	316
Accrued expenses and other current liabilities	(3,813)	(2,932)
Operating lease liabilities	(851)	(592)
Other liabilities	(1,156)	(644)
Net cash used in operating activities	(35,573)	(29,297)
Cash flows from investing activities
Purchases of marketable debt securities	(30,692)	(79,249)
Maturities of marketable debt securities	60,907	87,165
Sales of marketable equity securities	7,124
Purchases of property and equipment	(4,630)	(2,455)
Net cash provided by investing activities	32,709	5,461
Cash flows from financing activities
Proceeds from exercise of stock options	2,154	3,762
Proceeds from issuance of common stock under employee stock purchase plan	607	365
Net cash provided by financing activities	2,761	4,127
Net decrease in cash and cash equivalents and restricted cash	(103)	(19,709)
Cash and cash equivalents and restricted cash
Beginning of period	70,844	76,614
End of period	$ 70,741	$ 56,905

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2020

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation and Principles of Consolidation

The accompanying consolidated financial statements are unaudited and have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). The interim unaudited consolidated financial statements have been prepared on the same basis as the annual audited consolidated financial statements as of and for the year ended December 31, 2019 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019, which was filed with the SEC on February 26, 2020. Certain information and footnote disclosures required by GAAP which are normally included in the Company’s annual consolidated financial statements have been omitted pursuant to SEC rules and regulations for interim reporting. In the opinion of management, the accompanying consolidated financial statements reflect all adjustments, which are normal and recurring in nature, necessary for fair financial statement presentation.

The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year, any other interim periods, or any future year or period. These interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements as of and for the year ended December 31, 2019, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019.

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

Use of Estimates

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

The Company is actively monitoring the impact of the novel coronavirus (COVID-19) pandemic on its business, results of operations and financial condition. The full extent to which COVID-19 will directly or indirectly impact the Company’s business, results of operations and financial condition in the future is unknown at this time and will depend on future developments that are highly unpredictable. The most significant estimates affecting the Company’s consolidated financial statements that may be impacted by the COVID-19 pandemic are related to the Company’s assessment of credit losses on accounts receivable, contract assets and available-for-sale debt securities.

Reclassifications

Certain amounts reported in prior periods have been reclassified to conform to current period financial statement presentation. These reclassifications are not material and have no effect on previously reported financial position, results of operations and cash flows.

Restricted Cash

Restricted cash includes money market mutual funds used to collateralize irrevocable letters of credit as required by the Company’s lease agreements. The following table provides a reconciliation of cash and cash equivalents and restricted cash as reported on the consolidated balance sheets to the total of these amounts as reported at the end of the period in the consolidated statements of cash flows (in thousands):

	March 31, 2020		March 31, 2019
Cash and cash equivalents	$	69,411	$	55,852
Restricted cash		1,330		1,053
Total cash and cash equivalents and restricted cash	$	70,741	$	56,905

Accounts Receivable

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

Accounts receivable are stated net of an allowance for doubtful accounts, if deemed necessary based on the Company’s evaluation of collectability and potential credit losses. Management assesses the collectability of its accounts receivable using the specific identification of account balances, and considers the credit quality and financial condition of its significant customers, historical information regarding credit losses and the Company’s evaluation of current and expected future economic conditions. If necessary, an allowance for doubtful accounts is recorded against accounts receivable such that the carrying value of accounts

receivable reflects the net amount expected to be collected. The Company did not record an allowance for doubtful accounts as of March 31, 2020 and December 31, 2019.

Marketable Securities

Marketable securities consist of available-for-sale debt securities and equity securities and are carried at fair value. Marketable debt securities with remaining maturity dates exceeding 12 months which are not intended to be sold prior to maturity for use in current operations are classified as non-current assets. Marketable equity securities are classified as current assets.

Unrealized gains and losses on available-for-sale debt securities, net of any related tax effects, are excluded from results of operations and are included in other comprehensive income (loss) and reported as a separate component of stockholders’ equity until realized. The Company uses the aggregate portfolio approach to release the tax effects of unrealized gains and losses on available-for-sale debt securities in accumulated other comprehensive income (loss). Purchase premiums and discounts on marketable debt securities are amortized or accreted into the cost basis over the life of the related security as adjustments to the yield using the effective-interest method. Interest income is recognized when earned. Unrealized gains and losses on marketable equity securities are included in results of operations as investment income (loss). Realized gains and losses from the sale or maturity of marketable securities are based on the specific identification method and are included in results of operations as investment income (loss).

At each reporting date, the Company evaluates available-for-sale debt securities which have an amortized cost basis in excess of the fair value of the security to determine if the unrealized loss or any potential credit losses should be recognized in results of operations. If the Company does not have the intent and ability to hold the security until recovery of the unrealized loss, the difference between the fair value and amortized cost basis of the security is charged to results of operations resulting in a new amortized cost basis of the security. If the Company has the intent and ability to hold the security until recovery of the unrealized loss, the security is evaluated for potential credit losses. If a credit loss is deemed to exist, the credit loss is recognized in results of operations and an allowance for credit losses is recorded against the amortized cost basis of the security. In determining whether a credit loss exists related to impaired available-for-sale debt securities, the Company considers, among other factors, the extent of the unrealized loss relative to the amortized cost basis, the credit rating of the issuer and any recent changes thereto, current and expected future economic conditions, and any adverse events or other changes in circumstances that have occurred which may indicate a potential credit loss. The Company did not record an allowance for credit losses on its available-for-sale debt securities as of March 31, 2020.

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. Accounting Standards Codification (ASC) 820, Fair Value Measurements and Disclosures, 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 common stockholders by the weighted-average common shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is calculated by adjusting the weighted-average common shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury-stock method. Contingently convertible shares in which conversion is based on non-market-priced contingencies are excluded from the calculations of both basic and diluted net income loss per share until the contingency has been fully met. For purposes of the diluted net loss per share calculation, common stock equivalents are excluded from the calculation of diluted net loss per share if their effect would be anti-dilutive.

Recent Accounting Pronouncements

Recently Adopted Accounting Pronouncements

In June 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (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 that are not accounted for at fair value through net income be presented at the net amount expected to be collected by recording an allowance for credit losses. The 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 also amends the impairment model for available-for-sale debt securities, requiring credit losses on impaired debt securities to be included in results of operations. The Company adopted this standard effective January 1, 2020 using a modified retrospective transition method, which requires a cumulative-effect adjustment, if any, to opening accumulated deficit on the adoption date. The adoption of this standard primarily impacts the Company’s methodology used to assess credit losses on its accounts receivable, contract assets and available-for-sale debt securities. Based on the composition of the Company’s accounts receivable, contract assets and available-for-sale debt securities, the adoption of this standard required no cumulative-effect adjustments and did not have a material impact on the Company’s financial position or results of operations. Please refer to the significant accounting policies above for a description of the Company’s accounting policies for accounts receivable and marketable securities upon the adoption of this standard.

In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement, which modifies certain disclosure requirements regarding fair value measurements. The Company adopted this standard effective January 1, 2020. The adoption of this standard did not have a material impact on the Company’s financial statement disclosures.

In August 2018, the FASB issued ASU 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. The standard aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The Company adopted this standard effective January 1, 2020 on a prospective basis. The Company has various cloud-based software applications accounted for as service contracts, the most significant of which is the Company’s enterprise resource planning (ERP) system for which implementation was in progress on the adoption date of this standard. The adoption of this standard resulted in the capitalization of certain costs during the three months ended March 31, 2020 related to the implementation of the ERP system which would have been expensed as incurred prior to the adoption of this standard. The adoption of this standard did not have a material impact on the Company’s financial position or results of operations.

In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740)—Simplifying the Accounting for Income Taxes, which simplifies the current accounting for income taxes. Among other changes, the standard removes the exception to the incremental approach for intraperiod tax allocation when there is a loss from continuing operations and income or a gain from other items such as other comprehensive income. The Company early adopted this standard effective January 1, 2020, with certain aspects of the standard applied using the modified retrospective transition method and other aspects of the standard applied on a prospective basis. The adoption of this standard required no cumulative-effect adjustments and did not have a material impact on the Company’s financial position or results of operations.

Marketable Securities

3 Months Ended

Mar. 31, 2020

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 of available-for-sale debt securities and equity securities (in thousands):

	Amortized Cost / Cost		Unrealized Gains		Unrealized Losses			Fair Value
March 31, 2020
U.S. government and federal agency securities	$	49,791	$	378	$	—		$	50,169
Certificates of deposit		8,261		81		—			8,342
Corporate bonds		209,005		551		(730	)		208,826
Equity securities		282		19,588		—			19,870
	$	267,339	$	20,598	$	(730	)	$	287,207

	Amortized Cost / Cost		Unrealized Gains		Unrealized Losses			Fair Value
December 31, 2019
U.S. government and federal agency securities	$	62,637	$	215	$	(5	)	$	62,847
Certificates of deposit		8,506		77		—			8,583
Corporate bonds		226,137		808		(29	)		226,916
Equity securities		351		31,784		—			32,135
	$	297,631	$	32,884	$	(34	)	$	330,481

As of March 31, 2020 and December 31, 2019, no available-for-sale debt securities had remaining maturities greater than three years. The amortized cost of marketable debt securities is adjusted for amortization of premiums and accretion of discounts to maturity, or to the earliest call date for callable debt securities purchased at a premium.

As of March 31, 2020 and December 31, 2019, the balance in the Company’s accumulated other comprehensive income (loss) consisted solely of net unrealized gains and losses on available-for-sale debt securities, net of income tax effects and reclassification adjustments for realized gains and losses. During the three months ended March 31, 2020, the Company recognized net unrealized losses on available-for-sale debt securities of $0.8 million and income tax expense of zero in other comprehensive loss for the period. The Company recognized net realized gains of less than $0.1 million on the sale or maturity of available-for-sale debt securities during the three months ended March 31, 2020, which were reclassified out of accumulated other comprehensive income (loss) during the period and were included in investment income (loss) in the consolidated statements of operations and comprehensive loss. During the three months ended March 31, 2019, the Company recognized net unrealized gains on available-for-sale debt securities of $1.0 million and income tax expense of $0.4 million in other comprehensive income for the period. The Company recognized net realized gains of less than $0.1 million on the sale or maturity of available-for-sale debt securities during the three months ended March 31, 2019, which were reclassified out of accumulated other comprehensive income (loss) during the period and were included in investment income (loss) in the consolidated statements of operations and comprehensive loss.

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

	Less than 12 Months					12 Months or Greater				Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
March 31, 2020
Corporate bonds	$	90,352	$	(730	)	$	—	$	—	$	90,352	$	(730	)
	$	90,352	$	(730	)	$	—	$	—	$	90,352	$	(730	)

	Less than 12 Months					12 Months or Greater				Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
December 31, 2019
U.S. government and federal agency securities	$	12,562	$	(5	)	$	—	$	—	$	12,562	$	(5	)
Corporate bonds		48,556		(29	)		—		—		48,556		(29	)
	$	61,118	$	(34	)	$	—	$	—	$	61,118	$	(34	)

As of March 31, 2020, available-for-sale debt securities held by the Company which were in an unrealized loss position consisted of 30 investment grade security positions. The Company has the intent and ability to hold such securities until recovery, and due to the credit quality of the issuers and low severity of each unrealized loss position relative to its amortized cost basis, the Company has not identified any credit losses associated with its available-for-sale debt securities. The Company did not recognize any impairment or credit losses on available-for-sale debt securities during the three months ended March 31, 2020.

Marketable equity securities held by the Company as of March 31, 2020 consisted solely of common stock of Prevail Therapeutics Inc. (Prevail). The Company acquired the securities as consideration for a commercial license to the NAV Technology Platform granted to Prevail in August 2017. Prevail completed its initial public offering (IPO) in June 2019. Prior to Prevail’s IPO, the securities were accounted for as non-marketable equity securities without a readily determinable fair value and had a carrying value of $0.4 million. Upon Prevail’s IPO in June 2019, the securities were reclassified to marketable securities and are measured at fair value. During the three months ended March 31, 2020, the Company recognized unrealized losses of $12.2 million and realized gains of $7.1 million related to its marketable equity securities of Prevail, which were included in investment income (loss) in the consolidated statements of operations and comprehensive loss.

Fair Value of Financial Instruments

3 Months Ended

Mar. 31, 2020

Fair Value Disclosures [Abstract]

Fair Value of Financial Instruments

4. Fair Value of Financial Instruments

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

	Quoted		Significant
	prices		other		Significant
	in active		observable		unobservable
	markets		inputs		inputs
	(Level 1)		(Level 2)		(Level 3)		Total
March 31, 2020
Cash equivalents:
Money market mutual funds	$	—	$	54,530	$	—	$	54,530
Total cash equivalents		—		54,530		—		54,530
Marketable securities:
U.S. government and federal agency securities		—		50,169		—		50,169
Certificates of deposit		—		8,342		—		8,342
Corporate bonds		—		208,826		—		208,826
Equity securities		19,870		—		—		19,870
Total marketable securities		19,870		267,337		—		287,207
Total cash equivalents and marketable securities	$	19,870	$	321,867	$	—	$	341,737

	Quoted		Significant
	prices		other		Significant
	in active		observable		unobservable
	markets		inputs		inputs
	(Level 1)		(Level 2)		(Level 3)		Total
December 31, 2019
Cash equivalents:
Money market mutual funds	$	—	$	56,058	$	—	$	56,058
Total cash equivalents		—		56,058		—		56,058
Marketable securities:
U.S. government and federal agency securities		—		62,847		—		62,847
Certificates of deposit		—		8,583		—		8,583
Corporate bonds		—		226,916		—		226,916
Equity securities		32,135		—		—		32,135
Total marketable securities		32,135		298,346		—		330,481
Total cash equivalents and marketable securities	$	32,135	$	354,404	$	—	$	386,539

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

non-current portions are recorded at their present values using a discount rate that is based on prevailing market rates and the credit profile of the licensee on the date the amounts are initially recorded. Management does not believe there have been any significant changes in market conditions or credit quality that would cause the discount rates initially used to be significantly different from those that would be used as of March 31, 2020 to determine the present value of the receivables. Accordingly, management estimates that the carrying value of its non-current accounts receivable approximates the fair value of those instruments.

Non-marketable equity securities are measured at cost less impairment, adjusted for observable price changes for identical or similar investments of the same issuer. As of March 31, 2020 and December 31, 2019, the Company did not hold any non-marketable equity securities. No remeasurements or impairment losses were recorded on non-marketable equity securities during the three months ended March 31, 2020 and 2019.

- 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

License and Royalty Revenue

3 Months Ended

Mar. 31, 2020

License Agreement Revenue Recognition [Abstract]

License and Royalty Revenue

6. License and Royalty Revenue

As of March 31, 2020, the Company’s NAV Technology Platform was being applied by NAV Technology Licensees in one commercial product, Zolgensma, and in the development of more than 20 product candidates. Consideration to the Company under its license agreements may include: (i) up-front and annual fees, (ii) option fees to acquire additional licenses, (iii) milestone payments based on the achievement of certain development and sales-based milestones by licensees, (iv) sublicense fees and (v) royalties on sales of licensed products. Sublicense fees vary by license and range from a mid-single digit percentage to a low-double digit percentage of license fees received by licensees as a result of sublicenses. Royalties on net sales of commercialized products vary by license and range from a mid-single digit percentage to a low double-digit percentage of net sales by licensees.

Development milestone payments are evaluated each reporting period and are only included in the transaction price of each license and recognized as license revenue to the extent the milestones are considered probable of achievement. Sales-based milestones are excluded from the transaction price of each license agreement and recognized as royalty revenue in the period of achievement. As of March 31, 2020, the Company’s license agreements, excluding additional licenses that could be granted upon the exercise of options by licensees, contained unachieved milestones which could result in aggregate milestone payments to the Company of up to $371.4 million, including $0.3 million upon the submission of preclinical regulatory filings, $24.1 million upon the commencement of various stages of clinical trials, $31.0 million upon the submission of regulatory approval filings, $109.0 million upon the approval of commercial products by regulatory agencies and $207.0 million upon the achievement of specified sales targets for licensed products. To the extent the milestone payments are realized by the Company, the Company will be obligated to pay sublicense fees to licensors based on a specified percentage of the fees earned by the Company. The achievement of milestones by licensees is highly dependent on the successful development and commercialization of licensed products and it is at least reasonably possible that some or all of the milestone fees will not be realized by the Company.

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

	Balance at
	Beginning							Balance at
	of Period		Additions		Deductions			End of Period
Three Months Ended March 31, 2020
Receivables and contract assets:
Accounts receivable, current and non-current	$	42,303	$	18,142	$	(11,550	)	$	48,895
Contract assets	$	—	$	350	$	—		$	350
Contract liabilities:
Deferred revenue, current and non-current	$	3,333	$	—	$	—		$	3,333

	Balance at
	Beginning							Balance at
	of Period		Additions		Deductions			End of Period
Three Months Ended March 31, 2019
Receivables and contract assets:
Accounts receivable, current and non-current	$	31,599	$	1,738	$	(2,207	)	$	31,130
Contract assets	$	750	$	1,000	$	(750	)	$	1,000
Contract liabilities:
Deferred revenue, current and non-current	$	3,933	$	—	$	—		$	3,933

Additions to accounts receivable during the three months ended March 31, 2020 primarily consisted of royalties on net sales of Zolgensma of $10.0 million, receivables recorded related to new licenses granted during the period and interest income recognized during the period related to significant financing components. Additions to accounts receivable during the three months ended March 31, 2019 primarily consisted of amounts billed upon the achievement of development milestones by licensees and interest income recognized during the period related to significant financing components. Deductions to accounts receivable during the three months ended March 31, 2020 and 2019 primarily consisted of amounts collected from licensees during the period.

The changes in the balances of contract assets during the three months ended March 31, 2020 and 2019 consist of development milestones deemed probable of achievement by licensees during the period, offset by the subsequent achievement of such milestones and billing of associated milestone payments by the Company.

As of March 31, 2020, the Company had recorded deferred revenue of $3.3 million which represents consideration received from licensees for performance obligations that have not yet been satisfied by the Company. Unsatisfied performance obligations consist of options granted to licensees that provide material rights to the licensee to acquire additional licenses from the Company. These performance obligations will be satisfied, and underlying revenue will be recognized, upon the exercise or expiration of the options. The Company did not recognize any license revenue during the three months ended March 31, 2020 and 2019 that was included in deferred revenue at the beginning of the period.

During the three months ended March 31, 2020 and 2019, the Company recognized revenue of $10.4 million and $0.8 million, respectively, from performance obligations satisfied in prior periods as a result of changes in the transaction prices of its license agreements as well as royalties on sales of licensed products and sublicense fees. Changes in the transaction prices during the periods were primarily attributable to development milestones achieved or deemed probable of achievement during the period that were previously not considered probable of achievement.

As of March 31, 2020, the Company had recorded total current and non-current accounts receivable of $48.9 million, of which $7.1 million had been billed to customers and $41.8 million was billable to customers in future periods. As of December 31, 2019, the Company had recorded total current and non-current accounts receivable of $42.3 million, of which $0.4 million had been billed to customers and $41.9 million was billable to customers in future periods. Based on the Company’s evaluation of the credit quality and financial condition of its significant customers, history of collections and evaluation of current and future expected economic conditions, no credit losses were recognized on accounts receivable or contract assets during the three months ended March 31, 2020.

AveXis March 2014 License

In March 2014, the Company entered into an exclusive license agreement, as amended in January 2018 (the March 2014 License) with AveXis, Inc. (AveXis). Under the March 2014 License, the Company granted AveXis an exclusive, worldwide commercial license, with rights to sublicense, to the NAV Technology Platform, as well as other certain rights, for the treatment of spinal muscular atrophy (SMA) in humans by in vivo gene therapy. AveXis launched commercial sales of Zolgensma in the second quarter of 2019, which is a licensed product under the March 2014 License. Upon the commencement of commercial sales in the second quarter of 2019, the Company began recognizing royalty revenue on net sales of Zolgensma.

The Company recognized the following amounts under the March 2014 License with AveXis (in thousands):

	Three Months Ended March 31,
	2020		2019
License revenue	$	—	$	—
Zolgensma royalty revenue		9,978		—
Total license and royalty revenue	$	9,978	$	—

Interest income from licensing	$	7	$	8

As of March 31, 2020, the Company had recorded $10.0 million of accounts receivable from AveXis under the March 2014 License, of which $9.8 million were included in current assets and $0.2 million were included in non-current assets. As of December 31, 2019, the Company had recorded $11.0 million of accounts receivable from AveXis under the March 2014 License, of which $10.8 million were included in current assets and $0.2 million were included in non-current assets.

Abeona Therapeutics Inc.

Accounts receivable as of March 31, 2020 and December 31, 2019 included $26.9 million and $26.3 million, respectively, related to the November 2018 license agreement, as amended in November 2019 (the November 2018 License) with Abeona Therapeutics Inc. (Abeona) for the development and commercialization of treatments for various diseases, all of which were included in current assets.

Pursuant to the November 2018 License, Abeona was required to pay the Company a license fee of $8.0 million on April 1, 2020. In May 2020, after providing written notice to Abeona as required under the license agreement, the November 2018 License was effectively terminated by the Company as a result of breach by Abeona for the failure to pay the $8.0 million license fee due on April 1, 2020. Pursuant to the November 2018 License, Abeona is required to pay the aforementioned $8.0 million license fee to the Company immediately upon the termination of the license agreement. Additionally, Abeona is required to pay a $20.0 million license fee to the Company within 15 days of the termination date, which otherwise would have been due to the Company in November 2020.

Stock-based Compensation

3 Months Ended

Mar. 31, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-based Compensation

7. Stock-based Compensation

In January 2020, the Board of Directors authorized an additional 1,479,696 shares to be issued under the 2015 Equity Incentive Plan (the 2015 Plan). As of March 31, 2020, the total number of shares of common stock authorized for issuance under the 2015 Plan and the 2014 Stock Plan (the 2014 Plan) was 12,412,917, of which 2,454,658 remained available for future grants under the 2015 Plan.

Stock-based Compensation Expense

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

	Three Months Ended March 31,
	2020		2019
Stock options	$	7,779	$	5,452
Restricted stock units		—		68
Employee stock purchase plan		238		198
	$	8,017	$	5,718

As of March 31, 2020, the Company had $82.4 million of unrecognized stock-based compensation expense related to stock options and the 2015 Employee Stock Purchase Plan (the 2015 ESPP), which is expected to be recognized over a weighted-average period of 2.9 years.

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

	Three Months Ended March 31,
	2020		2019
Research and development	$	4,047	$	2,347
General and administrative		3,970		3,371
	$	8,017	$	5,718

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, 2019		5,544		$	28.79		7.5	$	86,509
Granted		1,261		$	39.12
Exercised		(194	)	$	11.64
Cancelled or forfeited		(173	)	$	44.35
Outstanding at March 31, 2020		6,438		$	30.91		7.8	$	52,998
Exercisable at March 31, 2020		3,202		$	20.33		6.5	$	49,710
Vested and expected to vest at March 31, 2020		6,438		$	30.91		7.8	$	52,998

(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, 2020 was $24.69. During the three months ended March 31, 2020, the total number of stock options exercised was 193,973, resulting in total proceeds of $2.3 million. The total intrinsic value of options exercised during the three months ended March 31, 2020 was $5.8 million.

Employee Stock Purchase Plan

In January 2020, the Board of Directors authorized an additional 369,924 shares to be issued under the 2015 ESPP. As of March 31, 2020, the total number of shares of common stock authorized for issuance under the 2015 ESPP was 623,924, of which 486,068 remained available for future issuance. During the three months ended March 31, 2020, 17,442 shares of common stock were issued under the 2015 ESPP.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2020

Accounting Policies [Abstract]

Basis of Presentation and Principles of Consolidation

Use of Estimates

Reclassifications

Restricted Cash

	March 31, 2020		March 31, 2019
Cash and cash equivalents	$	69,411	$	55,852
Restricted cash		1,330		1,053
Total cash and cash equivalents and restricted cash	$	70,741	$	56,905

Accounts Receivable

receivable reflects the net amount expected to be collected. The Company did not record an allowance for doubtful accounts as of March 31, 2020 and December 31, 2019.

Marketable Securities

Fair Value of Financial Instruments

•

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

•

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

•

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

Net Loss Per Share

Recent Accounting Pronouncements

Recently Adopted Accounting Pronouncements

Marketable Securities (Tables)

3 Months Ended

Mar. 31, 2020

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 of available-for-sale debt securities and equity securities (in thousands):

	Amortized Cost / Cost		Unrealized Gains		Unrealized Losses			Fair Value
March 31, 2020
U.S. government and federal agency securities	$	49,791	$	378	$	—		$	50,169
Certificates of deposit		8,261		81		—			8,342
Corporate bonds		209,005		551		(730	)		208,826
Equity securities		282		19,588		—			19,870
	$	267,339	$	20,598	$	(730	)	$	287,207

	Amortized Cost / Cost		Unrealized Gains		Unrealized Losses			Fair Value
December 31, 2019
U.S. government and federal agency securities	$	62,637	$	215	$	(5	)	$	62,847
Certificates of deposit		8,506		77		—			8,583
Corporate bonds		226,137		808		(29	)		226,916
Equity securities		351		31,784		—			32,135
	$	297,631	$	32,884	$	(34	)	$	330,481

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

	Less than 12 Months					12 Months or Greater				Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
March 31, 2020
Corporate bonds	$	90,352	$	(730	)	$	—	$	—	$	90,352	$	(730	)
	$	90,352	$	(730	)	$	—	$	—	$	90,352	$	(730	)

	Less than 12 Months					12 Months or Greater				Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
December 31, 2019
U.S. government and federal agency securities	$	12,562	$	(5	)	$	—	$	—	$	12,562	$	(5	)
Corporate bonds		48,556		(29	)		—		—		48,556		(29	)
	$	61,118	$	(34	)	$	—	$	—	$	61,118	$	(34	)

Fair Value of Financial Instruments (Tables)

3 Months Ended

Mar. 31, 2020

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, 2020
Cash equivalents:
Money market mutual funds	$	—	$	54,530	$	—	$	54,530
Total cash equivalents		—		54,530		—		54,530
Marketable securities:
U.S. government and federal agency securities		—		50,169		—		50,169
Certificates of deposit		—		8,342		—		8,342
Corporate bonds		—		208,826		—		208,826
Equity securities		19,870		—		—		19,870
Total marketable securities		19,870		267,337		—		287,207
Total cash equivalents and marketable securities	$	19,870	$	321,867	$	—	$	341,737

	Quoted		Significant
	prices		other		Significant
	in active		observable		unobservable
	markets		inputs		inputs
	(Level 1)		(Level 2)		(Level 3)		Total
December 31, 2019
Cash equivalents:
Money market mutual funds	$	—	$	56,058	$	—	$	56,058
Total cash equivalents		—		56,058		—		56,058
Marketable securities:
U.S. government and federal agency securities		—		62,847		—		62,847
Certificates of deposit		—		8,583		—		8,583
Corporate bonds		—		226,916		—		226,916
Equity securities		32,135		—		—		32,135
Total marketable securities		32,135		298,346		—		330,481
Total cash equivalents and marketable securities	$	32,135	$	354,404	$	—	$	386,539

License and Royalty Revenue (Tables)

3 Months Ended

Mar. 31, 2020

License Agreement Revenue Recognition [Abstract]

Summary of Changes in Balances of Receivables, Contract Assets and Contract Liabilities

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

	Balance at
	Beginning							Balance at
	of Period		Additions		Deductions			End of Period
Three Months Ended March 31, 2020
Receivables and contract assets:
Accounts receivable, current and non-current	$	42,303	$	18,142	$	(11,550	)	$	48,895
Contract assets	$	—	$	350	$	—		$	350
Contract liabilities:
Deferred revenue, current and non-current	$	3,333	$	—	$	—		$	3,333

	Balance at
	Beginning							Balance at
	of Period		Additions		Deductions			End of Period
Three Months Ended March 31, 2019
Receivables and contract assets:
Accounts receivable, current and non-current	$	31,599	$	1,738	$	(2,207	)	$	31,130
Contract assets	$	750	$	1,000	$	(750	)	$	1,000
Contract liabilities:
Deferred revenue, current and non-current	$	3,933	$	—	$	—		$	3,933

Schedule of License Revenue

The Company recognized the following amounts under the March 2014 License with AveXis (in thousands):

	Three Months Ended March 31,
	2020		2019
License revenue	$	—	$	—
Zolgensma royalty revenue		9,978		—
Total license and royalty revenue	$	9,978	$	—

Interest income from licensing	$	7	$	8

Stock-based Compensation (Tables)

3 Months Ended

Mar. 31, 2020

Stock-Based Compensation Expense by Award Type

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

	Three Months Ended March 31,
	2020		2019
Stock options	$	7,779	$	5,452
Restricted stock units		—		68
Employee stock purchase plan		238		198
	$	8,017	$	5,718

Stock-Based Compensation Expense

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

	Three Months Ended March 31,
	2020		2019
Research and development	$	4,047	$	2,347
General and administrative		3,970		3,371
	$	8,017	$	5,718

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, 2019		5,544		$	28.79		7.5	$	86,509
Granted		1,261		$	39.12
Exercised		(194	)	$	11.64
Cancelled or forfeited		(173	)	$	44.35
Outstanding at March 31, 2020		6,438		$	30.91		7.8	$	52,998
Exercisable at March 31, 2020		3,202		$	20.33		6.5	$	49,710
Vested and expected to vest at March 31, 2020		6,438		$	30.91		7.8	$	52,998

(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.

Marketable Securities - Additional Information (Detail)	3 Months Ended
Marketable Securities - Additional Information (Detail)	Mar. 31, 2020 USD ($) Security	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Available for sale debt securities remaining maturities greater than three years	$ 0		$ 0
Unrealized gains (losses) on available-for-sale debt securities, before tax	$ (800,000)	$ 1,000,000.0
Number of investment grade fixed income debt security \| Security	30
Other-than-temporary impaired	$ 0
Non-marketable equity securities, carrying value	400,000
Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Marketable securities, unrealized gain (loss)	(12,200,000)
Marketable securities, realized gain	7,100,000
Maximum [Member]
Schedule Of Available For Sale Securities [Line Items]
Unrealized gains (losses) on available-for-sale debt securities, income tax expense	0	400,000
Investment Income [Member] \| Maximum [Member]
Schedule Of Available For Sale Securities [Line Items]
Net realized gains (losses) available for sale securities	$ 100,000	$ 100,000

Marketable Securities - Summary of Fair Values and Unrealized Losses of Marketable Securities Held by the Company in an Unrealized Loss Position for Less Than 12 months and 12 Months or Greater (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Mar. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 90,352
Less than 12 Months, Unrealized Losses	(730)	$ (34)
Total, Fair Value	90,352
Total, Unrealized Losses	(730)	(34)
Less than 12 Months, Fair Value		61,118
Total, Fair Value		61,118
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	90,352
Less than 12 Months, Unrealized Losses	(730)	(29)
Total, Fair Value	90,352
Total, Unrealized Losses	$ (730)	(29)
Less than 12 Months, Fair Value		48,556
Total, Fair Value		48,556
U.S. Government and Federal Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Unrealized Losses		(5)
Total, Unrealized Losses		(5)
Less than 12 Months, Fair Value		12,562
Total, Fair Value		$ 12,562

Fair Value of Financial Instruments - Schedule of Fair Value of Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Marketable securities:
Fair Value	$ 287,207	$ 330,481
Cash Equivalents and Marketable Securities [Member]
Marketable securities:
Total cash equivalents and marketable securities	341,737	386,539
Corporate Bonds [Member]
Marketable securities:
Fair Value	208,826	226,916
Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	54,530	56,058
U.S. Government and Federal Agency Securities [Member]
Marketable securities:
Fair Value	50,169	62,847
Certificates of Deposit [Member]
Marketable securities:
Fair Value	8,342	8,583
Equity Securities [Member]
Marketable securities:
Fair Value	19,870	32,135
Money Market Mutual Funds [Member] \| Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	54,530	56,058
Quoted Prices in Active Markets (Level 1) [Member]
Marketable securities:
Fair Value	19,870	32,135
Quoted Prices in Active Markets (Level 1) [Member] \| Cash Equivalents and Marketable Securities [Member]
Marketable securities:
Total cash equivalents and marketable securities	19,870	32,135
Quoted Prices in Active Markets (Level 1) [Member] \| Equity Securities [Member]
Marketable securities:
Fair Value	19,870	32,135
Significant Other Observable Inputs (Level 2) [Member]
Marketable securities:
Fair Value	267,337	298,346
Significant Other Observable Inputs (Level 2) [Member] \| Cash Equivalents and Marketable Securities [Member]
Marketable securities:
Total cash equivalents and marketable securities	321,867	354,404
Significant Other Observable Inputs (Level 2) [Member] \| Corporate Bonds [Member]
Marketable securities:
Fair Value	208,826	226,916
Significant Other Observable Inputs (Level 2) [Member] \| Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	54,530	56,058
Significant Other Observable Inputs (Level 2) [Member] \| U.S. Government and Federal Agency Securities [Member]
Marketable securities:
Fair Value	50,169	62,847
Significant Other Observable Inputs (Level 2) [Member] \| Certificates of Deposit [Member]
Marketable securities:
Fair Value	8,342	8,583
Significant Other Observable Inputs (Level 2) [Member] \| Money Market Mutual Funds [Member] \| Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	$ 54,530	$ 56,058

License and Royalty Revenue - Additional Information (Detail)		3 Months Ended
	Apr. 01, 2020 USD ($)	Mar. 31, 2020 USD ($) ProductCandidate	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
License Revenue [Line Items]
Milestone fee upon submission of preclinical regulatory filings		$ 300,000
Milestone payment upon commencement of clinical trials in humans		24,100,000
Milestone payment upon submission of regulatory approval filings		31,000,000.0
Milestone payment upon approval of commercial products by regulatory agencies		109,000,000.0
Milestone payment upon achievement of specified sales targets for licensed products		207,000,000.0
Accounts receivable		48,895,000	$ 31,130,000	$ 42,303,000	$ 31,599,000
Deferred revenue, current and non-current		3,333,000	3,933,000	3,333,000	3,933,000
Licenses revenue		17,644,000	884,000
Credit losses on receivables or contract assets		0
Total current and non-current accounts receivable		48,895,000	31,130,000	42,303,000	$ 31,599,000
Billed to customers		7,100,000		400,000
Billable to customers in future periods		41,800,000		41,900,000
Accounts receivable, current		44,522,000		38,148,000
Abeona Therapeutics Inc. [Member] \| November 2018 License Agreement Termination [Member] \| Subsequent Event
License Revenue [Line Items]
Up-front and anuual fees	$ 20,000,000.0
March 2014 License Agreement [Member] \| AveXis, Inc. [Member]
License Revenue [Line Items]
Accounts receivable		10,000,000.0		11,000,000.0
Accounts receivable, current		9,800,000		10,800,000
Accounts receivable, non -current		200,000		200,000
November 2018 License Agreement [Member] \| Abeona Therapeutics Inc. [Member]
License Revenue [Line Items]
Accounts receivable		26,900,000		$ 26,300,000
November 2018 License Agreement [Member] \| Abeona Therapeutics Inc. [Member] \| Subsequent Event
License Revenue [Line Items]
License fee	8,000,000.0
November 2018 License Agreement [Member] \| Abeona Therapeutics Inc. [Member] \| November 2018 License Agreement Termination [Member] \| Subsequent Event
License Revenue [Line Items]
License fee	$ 8,000,000.0
License agreement termination claims, description	Additionally, Abeona is required to pay a $20.0 million license fee to the Company within 15 days of the termination date, which otherwise would have been due to the Company in November 2020.
Maximum [Member]
License Revenue [Line Items]
Aggregate milestone payment for all the targets		$ 371,400,000
N A V Technology Platform [Member]
License Revenue [Line Items]
Number of commercial product candidates \| ProductCandidate		1
N A V Technology Platform [Member] \| Minimum [Member]
License Revenue [Line Items]
Number of development partnered product candidates \| ProductCandidate		20
Zolgensma Royalties [Member]
License Revenue [Line Items]
Accounts receivable		$ 10,000,000.0
Total current and non-current accounts receivable		10,000,000.0
License [Member]
License Revenue [Line Items]
Licenses revenue		0
License [Member] \| License Agreement [Member]
License Revenue [Line Items]
License revenue		$ 10,400,000	$ 800,000

Stock-based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended
	Jan. 31, 2020	Mar. 31, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense		$ 82.4
Unrecognized stock-based compensation, weighted-average period		2 years 10 months 24 days
Proceeds from stock options exercised including unsettled options		$ 2.3
2015 Equity Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional shares to be issued	1,479,696
Common stock shares authorized for issuance		12,412,917
Shares available for future grants		2,454,658
Weighted-average fair values of options granted		$ 24.69
Exercise of stock options, Shares		193,973
Total intrinsic value of options exercised		$ 5.8
2015 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional shares to be issued	369,924
Common stock shares authorized for issuance		623,924
Shares available for future grants		486,068
Common stock shares issued to participants		17,442

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, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]
Shares Outstanding, Beginning Balance	5,544
Shares, Granted	1,261
Shares, Exercised	(194)
Shares, Cancelled or forfeited	(173)
Shares Outstanding, Ending Balance	6,438	5,544
Shares, Exercisable	3,202
Shares, Vested and expected to vest	6,438
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-average Exercise Price Outstanding, Beginning Balance	$ 28.79
Weighted-average Exercise Price, Granted	39.12
Weighted-average Exercise Price, Exercised	11.64
Weighted-average Exercise Price, Cancelled or forfeited	44.35
Weighted-average Exercise Price, Outstanding, Ending Balance	30.91	$ 28.79
Weighted-average Exercise Price, Exercisable	20.33
Weighted-average Exercise Price, Vested and expected to vest	$ 30.91
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average Remaining Contractual Life (Years) Outstanding	7 years 9 months 18 days	7 years 6 months
Weighted-average Remaining Contractual Life (Years), Exercisable	6 years 6 months
Weighted-average Remaining Contractual Life (Years), Vested and expected to vest	7 years 9 months 18 days
Aggregate Intrinsic Value Outstanding	$ 52,998	$ 86,509
Aggregate Intrinsic Value, Exercisable	49,710
Aggregate Intrinsic Value, Vested and expected to vest	$ 52,998

Nature of Business - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accumulated deficit	$ (217,860)	$ (177,822)
Cash, cash equivalents and marketable securities	$ 356,600

Property and Equipment Net - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Total property and equipment	$ 46,709	$ 43,311
Accumulated depreciation and amortization	(16,295)	(14,338)
Property and equipment, net	30,414	28,973
Lab Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	20,176	19,663
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	2,688	2,545
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	2,280	2,188
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	$ 21,565	$ 18,915

Income Taxes - Additional Information (Detail)	1 Months Ended
Income Taxes - Additional Information (Detail)	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Elimination of limitation in net operating losses deductions as percentage of current year taxable income, percentage	80.00%

Related Party Transactions - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
FOXKISER LLP [Member] \| Service Agreements [Member] \| Research and Development [Member]
Related Party Transaction [Line Items]
Related party transaction expense	$ 1.2	$ 0.8

Net Loss Per Share - Schedules for Computation of Diluted Weighted-Average Shares Outstanding (Detail) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	6,470	5,400
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	6,438	5,340
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	32	20

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Allowance for doubtful accounts receivable	$ 0	$ 0
Allowance for credit losses on available-for-sale debt securities	$ 0

Marketable Securities - Summary of Company Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost	$ 267,339	$ 297,631
Unrealized Gains	20,598	32,884
Unrealized Losses	(730)	(34)
Fair Value	287,207	330,481
U.S. Government and Federal Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost	49,791	62,637
Unrealized Gains	378	215
Unrealized Losses		(5)
Fair Value	50,169	62,847
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost	8,261	8,506
Unrealized Gains	81	77
Fair Value	8,342	8,583
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost	209,005	226,137
Unrealized Gains	551	808
Unrealized Losses	(730)	(29)
Fair Value	208,826	226,916
Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost	282	351
Unrealized Gains	19,588	31,784
Fair Value	$ 19,870	$ 32,135

License and Royalty Revenue - Summary of Changes in Balances of Receivables, Contract Assets and Contract Liabilities (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Receivables and contract assets:
Accounts receivable, current and non-current, Balance at Beginning of Period	$ 42,303	$ 31,599
Additions Related To Accounts Receivable	18,142	1,738
Deductions Related To Accounts Receivable	(11,550)	(2,207)
Accounts receivable, current and non-current, Balance at End of Period	48,895	31,130
Contract assets, Balance at Beginning of Period		750
Additions Related To Contract With Customer Assets	350	1,000
Deductions Related To Contract With Customer Asset		(750)
Contract assets, Balance at End of Period	350	1,000
Contract liabilities:
Deferred revenue, current and non-current, Balance at Beginning of Period	3,333	3,933
Deferred revenue, current and non-current, Balance at End of Period	$ 3,333	$ 3,933

Supplemental Disclosures - Schedules of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Payables And Accruals [Abstract]
Accrued personnel costs	$ 6,696	$ 10,903
Accrued sublicense fees and royalties	5,112	4,542
Accrued external research and development expenses	5,089	5,791
Accrued external general and administrative expenses	2,513	2,053
Accrued purchases of property and equipment	622	1,328
Other accrued expenses and current liabilities	295	229
Accrued expenses and other current liabilities	$ 20,327	$ 24,846