REGENXBIO (RGNX) 10-Q 7 May 19 2019 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	May 03, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
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 Common Stock, Shares Outstanding		36,636,825

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 55,852	$ 75,561
Marketable securities	229,373	244,200
Accounts receivable	8,372	8,587
Prepaid expenses	6,292	5,734
Other current assets	3,995	3,831
Total current assets	303,884	337,913
Marketable securities	159,083	150,819
Accounts receivable	22,758	23,012
Property and equipment, net	23,140	28,702
Operating lease right-of-use assets	6,858
Restricted cash	1,053	1,053
Other assets	2,255	2,315
Total assets	519,031	543,814
Current liabilities
Accounts payable	4,204	4,412
Accrued expenses and other current liabilities	14,189	17,164
Deferred revenue	600	600
Operating lease liabilities	2,397
Total current liabilities	21,390	22,176
Deferred revenue	3,333	3,333
Operating lease liabilities	5,483
Deferred rent		1,098
Financing lease obligations		5,854
Other liabilities	1,772	2,505
Total liabilities	31,978	34,966
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, 2019 and December 31, 2018
Common stock; $0.0001 par value; 100,000 shares authorized at March 31, 2019 and December 31, 2018; 36,611 and 36,120 shares issued and outstanding at March 31, 2019 and December 31, 2018, respectively	4	4
Additional paid-in capital	602,425	592,580
Accumulated other comprehensive loss	(59)	(720)
Accumulated deficit	(115,317)	(83,016)
Total stockholders’ equity	487,053	508,848
Total liabilities and stockholders’ equity	$ 519,031	$ 543,814

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
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	36,611,000	36,120,000
Common stock, shares outstanding	36,611,000	36,120,000

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenues
Revenues	$ 884	$ 132,391
Type of Revenue [Extensible List]	us-gaap:LicenseMember	us-gaap:LicenseMember
Costs of revenues
Costs of revenues	$ 29	$ 2,408
Type of Cost, Good or Service [Extensible List]	us-gaap:LicenseMember	us-gaap:LicenseMember
Research and development	$ 25,203	$ 19,550
General and administrative	11,558	8,380
Other operating expenses		28
Total operating expenses	36,790	30,366
Income (loss) from operations	(35,906)	102,025
Other Income
Interest income from licensing	613	1,355
Investment income	2,995	859
Total other income	3,608	2,214
Income (loss) before income taxes	(32,298)	104,239
Income Tax Benefit	70
Net income (loss)	(32,228)	104,239
Other Comprehensive Income (Loss)
Unrealized gain (loss) on available-for-sale securities, net of reclassifications and income tax expense	621	(188)
Total other comprehensive income (loss)	621	(188)
Comprehensive income (loss)	(31,607)	104,051
Net income (loss) applicable to common stockholders	$ (32,228)	$ 104,239
Net income (loss) per share:
Basic	$ (0.89)	$ 3.30
Diluted	$ (0.89)	$ 3.04
Weighted-average common shares outstanding:
Basic	36,366	31,632
Diluted	36,366	34,275

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional paid in capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Balances at Dec. 31, 2017	$ 183,029	$ 3	$ 371,497	$ (715)	$ (187,756)
Balances (Shares) at Dec. 31, 2017		31,295
Adoption of ASU | ASU 2014-09 [Member]	4,803				4,803
Exercise of stock options	3,824		3,824
Exercise of stock options, Shares		586
Issuance of common stock under employee stock purchase plan	342		342
Issuance of common stock under employee stock purchase plan, shares		20
Stock-based compensation expense	3,291		3,291
Unrealized gain (loss) on available-for-sale securities, net of reclassifications and income tax expense	(188)			(188)
Net income (loss)	104,239				104,239
Balances at Mar. 31, 2018	299,340	$ 3	378,954	(903)	(78,714)
Balances (Shares) at Mar. 31, 2018		31,900
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 [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 of reclassifications and income tax expense	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

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities
Net income (loss)	$ (32,228)	$ 104,239
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities
Stock-based compensation expense	5,718	3,291
Net amortization of premiums and accretion of discounts on marketable debt securities	(368)	378
Depreciation and amortization	1,614	834
Imputed interest income from licensing	(613)	(1,355)
Other non-cash adjustments	327	10
Changes in operating assets and liabilities
Accounts receivable	591	(51,414)
Prepaid expenses	(790)	667
Other current assets	(295)	(446)
Operating lease right-of-use assets	573
Other assets	26	(21)
Accounts payable	316	477
Accrued expenses and other current liabilities	(2,932)	490
Operating lease liabilities	(592)
Deferred rent		27
Other liabilities	(644)	957
Net cash provided by (used in) operating activities	(29,297)	58,134
Cash flows from investing activities
Purchases of marketable securities	(79,249)	(54,267)
Maturities of marketable securities	87,165	19,525
Purchases of property and equipment	(2,455)	(2,344)
Net cash provided by (used in) investing activities	5,461	(37,086)
Cash flows from financing activities
Proceeds from exercise of stock options	3,762	3,824
Proceeds from issuance of common stock under employee stock purchase plan	365	342
Net cash provided by financing activities	4,127	4,166
Net increase (decrease) in cash and cash equivalents and restricted cash	(19,709)	25,214
Cash and cash equivalents and restricted cash
Beginning of period	76,614	46,881
End of period	$ 56,905	$ 72,095

Nature of Business

Nature of Business	3 Months Ended
	Mar. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of Business	1. Nature of Business REGENXBIO Inc. (the Company) is a leading clinical-stage biotechnology company seeking to improve lives through the curative potential of gene therapy. The Company’s proprietary adeno-associated virus (AAV) gene delivery platform (NAV Technology Platform) consists of exclusive rights to over 100 novel AAV vectors, including AAV7, AAV8, AAV9 and AAVrh10. The Company’s NAV® Technology Platform is being applied by the Company, as well as by third-party licensees (NAV Technology Licensees), in the development of product candidates for a variety of diseases with unmet needs. The Company was formed in 2008 in the State of Delaware and is headquartered in Rockville, Maryland. Liquidity and Risks As of March 31, 2019, the Company had generated an accumulated deficit of $115.3 million since inception. As the Company has incurred cumulative losses since inception, transition to recurring profitability is dependent upon the successful development, approval and commercialization of its product candidates and achieving a level of revenues adequate to support the Company’s cost structure. The Company may never achieve recurring profitability, and unless and until it does, the Company will continue to need to raise additional capital. As of March 31, 2019, the Company had cash, cash equivalents and marketable securities of $444.3 million, which management believes is sufficient to fund operations for at least the next 12 months from the date these consolidated financial statements were issued. The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, development by the Company or its competitors of technological innovations, risks of failure of clinical trials, dependence on key personnel, protection of proprietary technology, compliance with government regulations and ability to transition from clinical manufacturing to the commercial production of products.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2019
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, 2018 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018, which was filed with the SEC on February 27, 2019. Certain information and footnote disclosures required by GAAP which are normally included in the Company’s annual consolidated financial statements have been omitted pursuant to SEC rules and regulations for interim reporting. In the opinion of management, the accompanying consolidated financial statements reflect all adjustments, which include all normal and recurring adjustments necessary for the fair statement of the Company’s financial position as of March 31, 2019, and the results of its operations and its cash flows for the interim periods ended March 31, 2019 and 2018. 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, 2018, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018. 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: revenue, stock-based compensation expense, accrued research and development expenses and other accrued liabilities, income taxes and the fair value of financial instruments. 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, 2019 March 31, 2018 Cash and cash equivalents $ 55,852 $ 71,870 Restricted cash 1,053 225 Total cash and cash equivalents and restricted cash $ 56,905 $ 72,095 Marketable Securities Marketable securities consist of debt securities and are classified as available-for-sale and carried at fair value. Marketable 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. Unrealized gains and losses, 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 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. 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. A decline in the fair value below cost of available-for-sale securities that is deemed other-than-temporary is charged to results of operations, resulting in the establishment of a new cost basis for the security. The Company regularly evaluates whether declines in the fair value of its investments below their cost are other-than-temporary. The evaluation includes consideration of the cause of the impairment, including the creditworthiness of the security issuers, the number of securities in an unrealized loss position, the severity and duration of the unrealized losses, whether the Company has the intent to sell the securities and whether it is more likely than not that the Company will be required to sell the securities before the recovery of their amortized cost basis. The Company has not recorded any impairment of available-for-sale securities which was deemed to be to be other-than-temporary. Leases Effective January 1, 2019, the Company adopted Accounting Standards Update (ASU) 2016-02, Leases Leases Under Topic 842, the Company applies a dual approach to all leases in which it is a lessee and classifies leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the Company. Lease classification is evaluated at the inception of the lease agreement. Regardless of classification, the Company records a right-of-use asset and a lease liability for all leases with a term greater than 12 months. All of the Company’s leases as of March 31, 2019 have been classified as operating leases. Operating lease expense is recognized on a straight-line basis over the term of the lease. Variable lease expense is recognized as incurred. The Company identifies leases in its contracts if the contract conveys the right to control the use of identified property, plant or equipment for a period of time in exchange for consideration. The Company does not allocate lease consideration between lease and nonlease components and records a lease liability equal to the present value of the remaining fixed consideration under the lease. The interest rates implicit in the Company’s leases are generally not readily determinable. Accordingly, the Company uses its estimated incremental borrowing rate at the commencement date of the lease to determine the present value discount of the lease liability. The Company estimates its incremental borrowing rate for each lease based on an evaluation of its expected credit rating and the prevailing market rates for collateralized debt in a similar economic environment with similar payment terms and maturity dates commensurate with the term of the lease. The right-of-use asset for each lease is equal to the lease liability, adjusted for unamortized initial direct costs and lease incentives and prepaid or accrued rent. Initial direct costs of entering into a lease are included in the right-of-use asset and amortized as lease expense over the term of the lease. Lease incentives, such as tenant improvements allowances, are recorded as a reduction of the right-of-use asset and amortized as a reduction of lease expense over the term of the lease. The Company excludes options to extend or terminate leases from the calculation of the lease liability unless it is reasonably certain the option will be exercised. 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. ASC 820, Fair Value Measurements and Disclosures • 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 Income (Loss) Per Share Basic net income (loss) per share is calculated by dividing net income (loss) applicable to common stockholders by the weighted-average common shares outstanding during the period, without consideration for common stock equivalents. Diluted net income (loss) per share is calculated by adjusting the weighted-average common shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury-stock method. Contingently convertible shares in which conversion is based on non-market-priced contingencies are excluded from the calculations of both basic and diluted net income (loss) per share until the contingency has been fully met. For purposes of the diluted net income (loss) per share calculation, common stock equivalents are excluded from the calculation of diluted net income (loss) per share if their effect would be anti-dilutive. Recent Accounting Pronouncements Adoption of ASU 2016-02, Leases In February 2016, the Financial Accounting Standards Board (FASB) issued ASU 2016-02, Leases Leases The Company elected certain practical expedients allowed by Topic 842 for transition purposes, including the package of practical expedients which permitted the Company to not reassess lease identification, classification and initial direct costs under Topic 842 for leases that commenced prior to January 1, 2019. Additionally, the Company elected the practical expedient allowed for transition purposes to use hindsight in determining the terms of leases that commenced prior to January 1, 2019. Upon the adoption of Topic 842, the Company recorded operating lease right-of-use assets of $7.4 million and operating lease liabilities of $8.4 million for its leases which were in effect and had commenced prior to January 1, 2019 and had original lease terms of more than 12 months. Additionally, upon the adoption of Topic 842, the Company derecognized $5.9 million of property and equipment and $5.9 million of financing lease obligations related to construction-in-progress at 9800 Medical Center Drive, as the Company does not control the building during the construction period under the requirements of Topic 842. The lease term for the facility at 9800 Medical Center Drive does not commence until certain construction is completed by the landlord and the building is delivered to the Company. The right-of-use assets and lease liabilities related to the facility at 9800 Medical Center Drive will not be recognized on the Company’s consolidated balance sheets until the commencement date of the lease, which is expected to occur in 2020. The cumulative impact of the adoption of Topic 842 resulted in an increase in accumulated deficit of less than $0.1 million on January 1, 2019. The adoption of Topic 842 did not have a material impact on the Company’s results of operations for the three months ended March 31, 2019, nor does the Company believe it will have a material impact on future results of operations based on its current leasing arrangements. Other recently adopted accounting pronouncements In February 2018, the FASB issued ASU 2018-02, Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income Tax Cuts and Jobs Act of 2017 (the TCJA) In April 2017, the FASB issued ASU 2017-08, Receivables—Nonrefundable Fees and Other Costs (Subtopic 310-20) Recent accounting pronouncements not yet adopted In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments

Marketable Securities

Marketable Securities	3 Months Ended
	Mar. 31, 2019
Investments Debt And Equity Securities [Abstract]
Marketable Securities	3. Marketable Securities The following tables present a summary of the Company’s marketable securities, which consist solely of available-for-sale debt securities (in thousands): Amortized Cost Unrealized Gains Unrealized Losses Fair Value March 31, 2019 U.S. government and federal agency securities $ 105,533 $ 202 $ (3 ) $ 105,732 Certificates of deposit 9,237 42 — 9,279 Corporate bonds 272,988 502 (45 ) 273,445 $ 387,758 $ 746 $ (48 ) $ 388,456 Amortized Cost Unrealized Gains Unrealized Losses Fair Value December 31, 2018 U.S. government and federal agency securities $ 103,410 $ 93 $ (37 ) $ 103,466 Certificates of deposit 8,992 — — 8,992 Corporate bonds 282,902 36 (377 ) 282,561 $ 395,304 $ 129 $ (414 ) $ 395,019 As of March 31, 2019 and December 31, 2018, 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, or to the earliest call date for callable debt securities purchased at a premium. As of March 31, 2019 and December 31, 2018, the balance in the Company’s accumulated other comprehensive loss consisted solely of net unrealized gains and losses on available-for-sale securities, net of income tax effects and reclassification adjustments for realized gains and losses. During the three months ended March 31, 2019, the Company recognized net unrealized gains on available-for-sale 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 securities during the three months ended March 31, 2019, which were reclassified out of accumulated other comprehensive loss during the period and are included in investment income in the consolidated statements of operations and comprehensive income (loss). During the three months ended March 31, 2018, the Company recognized net unrealized losses on available-for-sale securities of $0.2 million and income tax expense of zero in other comprehensive loss for the period. The Company did not recognize any realized gains or losses on the sale or maturity of available-for-sale securities during the three months ended March 31, 2018. Realized gains and losses from the sale or maturity of marketable securities are determined based on the specific identification method. The following tables present the fair values and unrealized losses of marketable securities held by the Company in an unrealized loss position for less than 12 months and 12 months or greater (in thousands): Less than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses March 31, 2019 U.S. government and federal agency securities $ 13,923 $ (3 ) $ — $ — $ 13,923 $ (3 ) Corporate bonds 110,103 (44 ) 1,999 (1 ) 112,102 (45 ) $ 124,026 $ (47 ) $ 1,999 $ (1 ) $ 126,025 $ (48 ) Less than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses December 31, 2018 U.S. government and federal agency securities $ 53,124 $ (37 ) $ — $ — $ 53,124 $ (37 ) Corporate bonds 245,283 (354 ) 12,424 (23 ) 257,707 (377 ) $ 298,407 $ (391 ) $ 12,424 $ (23 ) $ 310,831 $ (414 ) As of March 31, 2019, marketable securities held by the Company which were in an unrealized loss position consisted of 40 investment grade security positions. The Company has the intent and ability to hold such securities until recovery and has determined that none of its investments were other-than-temporarily impaired as of March 31, 2019 or December 31, 2018.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	3 Months Ended
	Mar. 31, 2019
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, 2019 Cash equivalents: Money market mutual funds $ — $ 55,834 $ — $ 55,834 Total cash equivalents — 55,834 — 55,834 Marketable securities: U.S. government and federal agency securities — 105,732 — 105,732 Certificates of deposit — 9,279 — 9,279 Corporate bonds — 273,445 — 273,445 Total marketable securities — 388,456 — 388,456 Total cash equivalents and marketable securities $ — $ 444,290 $ — $ 444,290 Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total December 31, 2018 Cash equivalents: Money market mutual funds $ — $ 75,542 $ — $ 75,542 Total cash equivalents — 75,542 — 75,542 Marketable securities: U.S. government and federal agency securities — 103,466 — 103,466 Certificates of deposit — 8,992 — 8,992 Corporate bonds — 282,561 — 282,561 Total marketable securities — 395,019 — 395,019 Total cash equivalents and marketable securities $ — $ 470,561 $ — $ 470,561 There were no transfers of financial instruments between levels of the fair value hierarchy during the three months ended March 31, 2019. 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, 2019 to determine the present value of the receivables. Accordingly, management estimates that the carrying value of its non-current accounts receivable approximates the fair value of those instruments. The Company’s non-marketable equity securities are measured at cost less impairment, adjusted for observable price changes for identical or similar investments of the same issuer. As of March 31, 2019 and December 31, 2018, non-marketable equity securities had carrying values of $0.4 million and were included in other assets on the consolidated balance sheets. Since the acquisition of the securities, the Company has not identified any observable price changes or changes in circumstances that would have had an adverse effect on the fair value of the securities as of March 31, 2019 and December 31, 2018. No remeasurements or impairment losses were recorded on non-marketable equity securities during the three months ended March 31, 2019 and 2018.

Property and Equipment, Net

Property and Equipment, Net	3 Months Ended
	Mar. 31, 2019
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, 2019 December 31, 2018 Lab equipment $ 15,563 $ 14,417 Computer equipment and software 2,104 2,002 Furniture and fixtures 1,935 1,915 Leasehold improvements 12,371 11,751 Construction-in-progress — 5,854 Total property and equipment 31,973 35,939 Accumulated depreciation and amortization (8,833 ) (7,237 ) Property and equipment, net $ 23,140 $ 28,702 Construction-in-progress reported in the table above as of December 31, 2018 consisted of certain costs incurred and reported by the Company’s landlord at 9800 Medical Center Drive. Upon the adoption of Topic 842 on January 1, 2019, the Company derecognized the cumulative amount of construction costs incurred by the landlord of $5.9 million. Please refer to Note 2 for further information on the Company’s adoption of Topic 842 and Note 6 for further information on the Company’s lease agreement for the facility at 9800 Medical Center Drive.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	6. Commitments and Contingencies Lease Commitments 9800 Medical Center Drive Lease In November 2018, the Company entered into a lease for approximately 132,000 square feet of office and laboratory facilities in a new building to be constructed at 9800 Medical Center Drive in Rockville, Maryland (the 9800 Medical Center Drive Lease). Construction of the new building, which will be conducted by the landlord, is expected to be completed in 2020 and the lease will expire approximately 16 years from the delivery of the leased premises to the Company, subject to certain extension and termination options. Under the original terms of the 9800 Medical Center Drive Lease, which was amended in April 2019 as discussed further below, the Company was entitled to receive a $14.6 million tenant improvement allowance from the landlord to construct additional improvements to the leased premises. The Company has the option to extend the term of the lease for up to 10 additional years and the option to terminate the lease after 12 years from the delivery of the leased premises to the Company. If the Company elects to terminate the lease, it will be subject to a termination fee equal to the unamortized tenant improvement allowance, rent abatement and landlord commissions as of the termination date, bearing interest at 5% per annum, plus four months of base rent and operating expenses. Additionally, after delivery of the leased premises under the 9800 Medical Center Drive Lease, the Company will have the option to terminate its lease at 9712 Medical Center Drive with six months’ notice. Monthly payments under the 9800 Medical Center Drive Lease begin approximately 12 months from the delivery of the leased premises to the Company and escalate annually in accordance with the lease agreement. As required by the lease agreement, the Company has provided the landlord with an irrevocable letter of credit of $0.8 million which the landlord may draw upon in the event of any uncured default by the Company under the terms of the lease. The Company is involved in the construction project for the leased premises at 9800 Medical Center Drive, including having the responsibility to pay for a portion of the costs of non-normal tenant improvements such as finish work, mechanical, electrical and plumbing elements of the building, among other items. As of December 31, 2018, under the requirements of Topic 840, the Company was deemed the owner of the leased premises during the construction period for accounting purposes and certain estimated construction costs incurred and reported by the landlord were recorded as property and equipment, with a corresponding financing lease obligation, on the consolidated balance sheet. The Company has determined that it does not control the building during the construction period under the requirements of Topic 842. Accordingly, upon the adoption of Topic 842 on January 1, 2019, the Company derecognized the property and equipment of $5.9 million for the cumulative costs of construction incurred by the landlord as well as the associated $5.9 million financing lease obligation. As of March 31, 2019, the Company had incurred $0.3 million of costs related to construction-in-progress at 9800 Medical Center Drive, which have been recorded as leasehold improvements within property and equipment on the consolidated balance sheets. In April 2019, the Company agreed to pay $4.0 million to the landlord to fund certain costs of construction related to material changes in the design and construction of the building requested by the Company. The right-of-use assets and lease liabilities related to the 9800 Medical Center Drive Lease have not been recorded on the Company’s consolidated balance sheets as of March 31, 2019 and will be measured and recognized on the commencement date of the lease, which is expected to occur in 2020 when the landlord delivers the newly constructed facility to the Company. In April 2019, the Company amended the 9800 Medical Center Drive Lease to expand the leased premises to include an additional 5,975 square feet of the building over the term of the lease. As a result of the amendment, the total amount of future rent under the 9800 Medical Center Drive Lease was increased by $4.0 million and the Company’s tenant improvement allowance under the lease was increased to $15.3 million. Other Lease Commitments In March 2015, the Company entered into a non-cancelable operating lease for office space at 9712 Medical Center Drive in Rockville, Maryland (the 9712 Medical Center Drive Lease). The lease term commenced in April 2015. Monthly payments under the lease began in October 2015 and escalate annually in accordance with the lease agreement. In September 2015, November 2015, July 2017 and April 2018, the Company amended the 9712 Medical Center Drive Lease to include additional office and laboratory space at 9714 Medical Center Drive, and ultimately extend the term of the lease to September 2021. The Company has options to extend the term of the 9712 Medical Center Drive Lease for up to six additional years. Additionally, upon the commencement of the 9800 Medical Center Drive Lease, the Company will have the option to terminate the 9712 Medical Center Drive Lease with six months’ notice. The Company’s extension and termination options under the 9712 Medical Center Drive Lease have been excluded from the measurement of the right-of-use assets and lease liabilities for the lease as they are not reasonably certain of exercise. The Company received a $0.4 million tenant improvement allowance from the landlord which has been recorded as a reduction of the right-of-use assets for the lease and is amortized on a straight-line basis as a reduction of rent expense over the term of the lease. In January 2016, the Company entered into a 7.5-year, non-cancelable operating lease for its corporate headquarters at 9600 Blackwell Road in Rockville, Maryland (the Blackwell Road Lease). The lease commenced in February 2016 and expires in September 2023. The Company has an option to extend the term of the Blackwell Road Lease for up to five additional years and the option to terminate the lease after 67 months from the lease commencement date. If the Company elects to terminate the lease, it will be subject to a termination fee equal to the unamortized tenant improvement allowance, rent abatement and landlord costs and commissions as of the termination date, bearing interest at 8% per annum. The Company’s extension and termination options under the Blackwell Road Lease have been excluded from the measurement of the right-of-use assets and lease liabilities for the lease as they are not reasonably certain of exercise. In November 2017, the Blackwell Road Lease was amended to include additional office space for the remainder of the lease term. Monthly payments under the lease began in September 2016 and escalate annually in accordance with the lease agreement. The Company received a $0.8 million tenant improvement allowance from the landlord which has been recorded as a reduction of the right-of-use assets for the lease and is amortized on a straight-line basis as a reduction of rent expense over the term of the lease. The Company leases additional office and laboratory facilities in Rockville, Maryland and New York, New York, as well as laboratory and other equipment, under non-cancelable operating leases with various expiration dates through 2022 and which may contain annual escalations of rental payments. As required by the Company’s lease agreement for its office space in New York, New York, 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. All of the Company’s leases are classified as operating leases. The following table summarizes the Company’s lease costs and supplemental cash flow information related to operating leases (in thousands): Three Months Ended March 31, 2019 Operating lease cost $ 700 Variable lease cost 161 Total lease cost $ 861 Cash paid for amounts included in the measurement of operating lease liabilities $ 711 Right-of-use assets acquired through operating lease liabilities $ 36 Variable lease cost under the Company’s operating leases includes items such as common area maintenance, utilities, taxes and other charges. The weighted-average remaining lease term and weighted-average discount rate of the Company’s operating leases were as follows: As of March 31, 2019 Weighted-average remaining lease term (years) 3.1 Weighted-average discount rate 5.6 % The following table presents a reconciliation of the undiscounted future minimum lease payments remaining under the 9800 Medical Center Drive Lease and other operating leases to the amounts reported as operating lease liabilities on the consolidated balance sheet as of March 31, 2019 (in thousands): 9800 Medical Other Total Center Drive Operating Undiscounted Lease (a) Leases Fixed Payments Undiscounted future minimum lease payments: 2019 (remainder of year) $ — $ 1,995 $ 1,995 2020 — 3,088 3,088 2021 1,332 2,412 3,744 2022 4,308 623 4,931 2023 5,188 479 5,667 Thereafter 76,892 — 76,892 Total undiscounted future minimum lease payments $ 87,720 $ 8,597 $ 96,317 Amount representing imputed interest (717 ) Total operating lease liabilities 7,880 Current portion of operating lease liabilities (2,397 ) Operating lease liabilities, non-current $ 5,483 (a) Undiscounted future minimum lease payments under the 9800 Medical Center Drive Lease are not included in the lease liabilities reported on the consolidated balance sheet as of March 31, 2019 as the lease has not yet commenced. The actual timing and amounts of payments under the 9800 Medical Center Drive Lease are subject to adjustment based on the completion date of construction and actual square footage of the facility constructed. Accordingly, these amounts were estimates as of March 31, 2019. As of December 31, 2018, future minimum lease payments under Topic 840 for the 9800 Medical Center Drive Lease and other operating leases were as follows (in thousands): 9800 Medical Other Total Center Drive Operating Minimum Lease Lease (a) Leases Payments 2019 $ — $ 2,798 $ 2,798 2020 — 3,054 3,054 2021 1,329 2,391 3,720 2022 4,289 621 4,910 2023 5,156 479 5,635 Thereafter 76,420 — 76,420 Total minimum lease payments $ 87,194 $ 9,343 $ 96,537 (a) Includes all future minimum lease payments under the 9800 Medical Center Drive Lease, including amounts recorded as financing lease obligations on the consolidated balance sheet . The actual timing and amounts of payments under the 9800 Medical Center Drive Lease are subject to adjustment based on the completion date of construction and actual square footage of the facility constructed. Accordingly, these amounts were estimates as of December 31, 2018. License from The Trustees of the University of Pennsylvania In February 2009, the Company entered into a license agreement, which has been amended from time to time, with The Trustees of the University of Pennsylvania (together with the University of Pennsylvania, Penn) for exclusive, worldwide rights to certain patents owned by Penn underlying the Company’s NAV Technology Platform, as well as exclusive rights to certain data, results and other information generated in connection with the clinical trial for RGX-501, the Company’s product candidate for the treatment of homozygous familial hypercholesterolemia (HoFH). In consideration for the license, the Company issued Penn an equity interest in the Company and is obligated to pay Penn royalties on net sales of licensed products and sublicense fees. Additionally, the Company is obligated to reimburse Penn for certain costs incurred related to the maintenance of the licensed patents. In April 2019, the Company amended its license from Penn to include exclusive license rights to certain know-how, including research data and other information, relating to the treatment of late-infantile neuronal ceroid lipofuscinosis type 2 (CLN2) disease. In consideration for the additional licensed rights, and in addition to any consideration owed under the license prior to the amendment, the Company is obligated to pay Penn an upfront fee, milestone fees of up to $20.5 million upon the achievement of various development and sales-based milestones and additional royalties on net sales of licensed products for the treatment of CLN2 disease. Additionally, the amendment modifies the percentage of sublicense fees the Company is obligated to pay Penn on amounts the Company receives from third parties for the sublicensing of the licensed rights for the treatment of CLN2 disease. European Patent Office Proceeding In June 2017, a third party filed an opposition with the European Patent Office (EPO) challenging the validity of a European patent owned by Penn for the AAV8 vector, which the Company has exclusively licensed (EU AAV8 Patent). The EPO conducted oral proceedings in October 2018 and upheld the validity of the EU AAV8 Patent subject to certain amendments made during the proceeding. Each party to the proceeding has appealed the EPO’s ruling. As of March 31, 2019 and December 31, 2018, the Company had not recorded any liabilities related to this matter nor does the Company believe this matter will have a material adverse impact on its business.

License Revenue

License Revenue	3 Months Ended
	Mar. 31, 2019
License Agreement Revenue Recognition [Abstract]
License Revenue	7. License Revenue As of March 31, 2019, the Company’s NAV Technology Platform was being applied in the development of more than 20 partnered product candidates by 11 NAV Technology Licensees. 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. To date the Company has not recognized any revenue from the achievement of sales-based milestones or royalties on sales of licensed products. Development milestone payments are only included in the transaction price of each license and recognized as revenue to the extent they are considered probable of achievement. Sales-based milestones are excluded from the transaction price of each license agreement and recognized as revenue in the period of achievement. As of March 31, 2019, 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 $29.6 million upon the commencement of various stages of clinical trials, $47.5 million upon the submission of regulatory approval filings, $117.5 million upon the approval of commercial products by regulatory agencies and $232.0 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 Net Additions Balance at of Period (Deductions) End of Period Three Months Ended March 31, 2019 Receivables and contract assets: Accounts receivable, current and non-current $ 31,599 $ (469 ) $ 31,130 Contract assets $ 750 $ 250 $ 1,000 Contract liabilities: Deferred revenue, current and non-current $ 3,933 $ — $ 3,933 Balance at Beginning Net Additions Balance at of Period (Deductions) End of Period Three Months Ended March 31, 2018 Receivables and contract assets: Accounts receivable, current and non-current $ 5,850 $ 52,762 $ 58,612 Contract assets $ 350 $ (100 ) $ 250 Contract liabilities: Deferred revenue, current and non-current $ — $ — $ — The net increase in the balance of accounts receivable during the three months ended March 31, 2018 was primarily attributable to the January 2018 amendment to the license agreement with AveXis, Inc. (AveXis) for the development and commercialization of treatments for spinal muscular atrophy (SMA). As of March 31, 2018, the Company had recorded accounts receivable, current and non-current, of $53.5 million related to the amended license agreement with AveXis. As of March 31, 2019, the Company had recorded deferred revenue, current and non-current, of $3.9 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. During the three months ended March 31, 2019 and 2018, the Company recognized license revenue of $0.8 million and $0.3 million, respectively, from licenses delivered to licensees in prior periods as a result of changes in the transaction prices of its license agreements. Changes in the transaction price were primarily attributable to development milestones achieved or deemed probable of achievement during the period that were previously not considered probable of achievement. As of March 31, 2019, the Company had recorded total current and non-current accounts receivable of $31.1 million, of which $0.1 million had been billed to customers and $31.0 million was billable to customers in future periods. As of December 31, 2018, the Company had recorded total current and non-current accounts receivable of $31.6 million, of which $0.4 million had been billed to customers and $31.2 million was billable to customers in future periods. Accounts receivable, current and non-current, as of March 31, 2019 and December 2018 included $26.7 million and $26.0 million, respectively, related to the November 2018 license agreement with Abeona Therapeutics Inc. for the development and commercialization of treatments for various diseases. The Company believes that it is not exposed to significant credit risk related to accounts receivable due to the credit quality and history of collections from its significant customers. The Company is unaware of any concentrations of credit risk related to accounts receivable from significant customers with deteriorated credit quality. As of March 31, 2019 and December 31, 2018, the Company had not recognized any impairment losses on its receivables or contract assets from contracts with customers. AveXis March 2014 License and January 2018 Amendment In March 2014, the Company entered into an exclusive license agreement (the March 2014 License) with AveXis. Under the license, the Company granted AveXis an exclusive, worldwide commercial license, with rights to sublicense, to the NAV AAV9 vector for the treatment of SMA in humans by in vivo gene therapy. mid-single to low double-digit royalties on net sales of licensed products, In January 2018, the Company and AveXis amended the March 2014 License (the January 2018 Amendment). Under the January 2018 Amendment, the licensed intellectual property was expanded to include, in addition to the NAV AAV9 vector previously licensed, sublicenses to other third-party patents exclusively licensed by the Company as well as any other recombinant AAV vector in the Company’s intellectual property portfolio during a period of 14 years from the effective date of the January 2018 Amendment, for the treatment of SMA in humans by in vivo The January 2018 Amendment also modified the assignment provision of the March 2014 License. Under the amended assignment provision, AveXis was permitted to transfer the March 2014 License, as amended, without the Company’s consent in connection with a change of control of AveXis, subject to certain conditions. Under the original March 2014 License, any assignment by AveXis without the Company’s prior written consent had been prohibited. In consideration for the additional rights granted under the January 2018 Amendment, and in addition to any consideration owed under the original March 2014 License, AveXis paid to the Company a fee of $80.0 million upon entry into the January 2018 Amendment. In addition, AveXis was obligated to pay the Company (i) $30.0 million on the first anniversary of the effective date of the January 2018 Amendment, (ii) $30.0 million on the second anniversary of the effective date of the January 2018 Amendment and (iii) potential sales-based milestone payments of up to $120.0 million. In the event of a change of control of AveXis, to the extent that any fee described in (i) or (ii) above, or the first $40.0 million of sales-based milestone payments described in (iii) above, had not yet been paid to the Company, AveXis was required to pay any such unpaid fee to the Company upon the change of control. For any product developed for the treatment of SMA using the NAV AAV9 vector, AveXis will continue to be obligated to pay to the Company mid-single to low double-digit royalties on net sales as required by the March 2014 License, and for any product developed for the treatment of SMA using a licensed vector other than NAV AAV9, the Company will receive a low double-digit royalty on net sales. In May 2018, AveXis was acquired by Novartis AG (Novartis), which qualified as a change of control of AveXis under the January 2018 Amendment. Pursuant to the January 2018 Amendment, AveXis paid the Company $100.0 million in accelerated license payments as a result of the change of control. Accounting Analysis The January 2018 Amendment was accounted for under Topic 606 as a modification of the license agreement resulting in a new and separate contract from the original March 2014 License for revenue recognition purposes. The Company determined that a substantive termination penalty is associated with AveXis’ termination rights under the amended license agreement, and therefore the contract term for revenue recognition purposes is equal to the stated term of the license. The only material performance obligation of the Company under the January 2018 Amendment is for the delivery of the modified license, which occurred upon the execution of the amendment in January 2018. As of March 31, 2019, the transaction price of the original March 2014 License was $7.5 million. The transaction price includes (i) the up-front payment in March 2014 of $2.0 million, (ii) the present value of aggregate annual fees payable to the Company over the term of the license and (iii) payments for development milestones achieved to date or which are deemed probable of achievement. The discounted portion of the annual fees represents the financing benefit provided to AveXis and is recognized as interest income from licensing over the term of the license. Variable consideration under the original March 2014 License, which has been excluded from the transaction price, includes $7.0 million in payments for remaining development milestones that had not yet been achieved and were not considered probable of achievement, as well as any potential sublicense fees or royalties on sales of licensed products, which will be recognized in the period of the underlying sales or sublicenses, if any. Upon its execution, the transaction price of the January 2018 Amendment was $132.1 million, which was fully recognized as license revenue upon the delivery of the modified license in January 2018. In May 2018, as a result of the acquisition of AveXis by Novartis, the transaction price was increased by $40.0 million to account for the acceleration of the sale-based milestone which was previously excluded from the transaction price. The $40.0 million increase in the transaction price was recognized as license revenue upon the completion of the change of control in May 2018 since the amended license had been fully delivered to AveXis. Additionally, due to the acceleration of the two $30.0 million payments originally due in January 2019 and January 2020, the Company recognized $6.1 million of interest income from licensing upon the completion of the change of control of AveXis, which represents the remaining present value discount on such payments as of the date of the change of control. As of March 31, 2019, the transaction price of the January 2018 Amendment was $172.1 million, which includes: (i) the $80.0 million payment in January 2018, (ii) the present value, as of the date of the January 2018 Amendment, of the two $30.0 million payments originally due in January 2019 and January 2020 and (iii) the $40.0 million sales-based milestone which was accelerated upon the change of control in May 2018. Variable consideration under the January 2018 Amendment, which has been excluded from the transaction price, includes the remaining sales-based milestone payment of $80.0 million, as well as any potential sublicense fees or royalties on sales of licensed products, which will be recognized in the period of the underlying sales or sublicenses, if any. During the three months ended March 31, 2019 and 2018, the Company recognized license revenue of zero and $132.1 million, respectively, and interest income from licensing of less than $0.1 million and $1.2 million, respectively, from the March 2014 License, as amended, which includes amounts from both the original March 2014 License and the January 2018 Amendment. As of March 31, 2019 and December 31, 2018, the Company had recorded $0.2 million of accounts receivable from AveXis under the March 2014 License, as amended, of which less than $0.1 million were included in current assets and $0.2 million were included in non-current assets on the consolidated balance sheets.

Stock-based Compensation

Stock-based Compensation	3 Months Ended
	Mar. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based Compensation	8. Stock-based Compensation In January 2019, an additional 1,444,808 shares became available for issuance under the 2015 Equity Incentive Plan (the 2015 Plan). As of March 31, 2019, the total number of shares of common stock authorized for issuance under the 2015 Plan and the 2014 Stock Plan (the 2014 Plan) was 10,933,221, of which 2,583,144 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, 2019 2018 Stock options $ 5,452 $ 3,122 Restricted stock units 68 68 Employee stock purchase plan 198 101 $ 5,718 $ 3,291 As of March 31, 2019, the Company had $68.9 million of unrecognized stock-based compensation expense related to stock options, restricted stock units and the 2015 Employee Stock Purchase Plan (the 2015 ESPP), which is expected to be recognized over a weighted-average period of 3.1 years. The Company has recorded aggregate stock-based compensation expense in the consolidated statements of operations and comprehensive income (loss) as follows (in thousands): Three Months Ended March 31, 2019 2018 Research and development $ 2,347 $ 1,530 General and administrative 3,371 1,761 $ 5,718 $ 3,291 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, 2018 4,855 $ 19.31 7.6 $ 118,185 Granted 1,102 $ 48.22 Exercised (480 ) $ 7.81 Cancelled or forfeited (137 ) $ 30.72 Outstanding at March 31, 2019 5,340 $ 26.02 7.9 $ 170,518 Exercisable at March 31, 2019 2,491 $ 12.22 6.8 $ 112,324 Vested and expected to vest at March 31, 2019 5,340 $ 26.02 7.9 $ 170,518 (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, 2019 was $32.20. During the three months ended March 31, 2019, the total number of stock options exercised was 480,320, resulting in total proceeds of $3.8 million. The total intrinsic value of options exercised during the three months ended March 31, 2019 was $20.0 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, 2018 40 $ 20.90 Granted — $ — Vested — $ — Forfeited — $ — Unvested balance at March 31, 2019 40 $ 20.90 Employee Stock Purchase Plan As of March 31, 2019, the total number of shares of common stock authorized for issuance under the 2015 ESPP was 254,000, of which 159,339 remained available for future issuance. During the three months ended March 31, 2019, 10,241 shares of common stock were issued under the 2015 ESPP.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	9. Income Taxes The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets. Based on the Company’s history of operating losses, including a three-year cumulative loss position as of March 31, 2019 and December 31, 2018, the Company has concluded that it is more likely than not that the benefit of its deferred tax assets will not be realized. Accordingly, the Company has provided a full valuation allowance for its net deferred tax assets as of March 31, 2019 and December 31, 2018. During the three months ended March 31, 2019, the Company recognized income tax benefit of $0.1 million and income tax expense in other comprehensive income of $0.4 million related to net unrealized gains on available-for-sale securities during the period. As of March 31, 2019, the Company had accrued $0.3 million related to this tax benefit, which is expected to be generated from losses in continuing operations in 2019 and is included in accrued expenses and other current liabilities on the consolidated balance sheets.

Related Party Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2019
Related Party Transactions [Abstract]
Related Party Transactions	10. Related Party Transactions FOXKISER LLP Since 2016, the Company has been party to professional services agreements with FOXKISER LLP (FOXKISER), an affiliate of certain stockholders of the Company and an affiliate of a member of the Company’s Board of Directors, pursuant to which the Company pays a fixed monthly fee in consideration for certain strategic services provided by FOXKISER. Effective January 2019, the Company entered into a new professional services agreement with FOXKISER with similar terms and conditions as the previous agreements and which has a term of one year and is terminable by either party. Expenses incurred under the agreements with FOXKISER for the three months ended March 31, 2019 and 2018 were $0.8 million and $0.5 million, respectively, and were recorded as research and development expenses in the consolidated statements of operations and comprehensive income (loss).

Net Income (Loss) Per Share

Net Income (Loss) Per Share	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Net Income (Loss) Per Share	11. Net Income (Loss) Per Share The computations of basic and diluted net income (loss) per share are as follows (in thousands, except per share data): Three Months Ended March 31, 2019 2018 Basic net income (loss) per share: Net income (loss) applicable to common stockholders $ (32,228 ) $ 104,239 Shares used in computation: Weighted-average common shares outstanding 36,366 31,632 Basic net income (loss) per share $ (0.89 ) $ 3.30 Diluted net income (loss) per share: Net income (loss) applicable to common stockholders $ (32,228 ) $ 104,239 Shares used in computation: Weighted-average common shares outstanding 36,366 31,632 Stock options — 2,620 Restricted stock units — 22 Employee stock purchase plan — 1 Weighted-average diluted common shares 36,366 34,275 Diluted net income (loss) per share $ (0.89 ) $ 3.04 For periods in which the Company incurred net losses applicable to common stockholders, common stock equivalents are excluded from the calculation of diluted net loss per share as their effect would be anti-dilutive, and accordingly, basic and diluted net loss per share are the same for such periods. Outstanding stock options with exercise prices greater than the average market price of common stock are excluded from the calculation of diluted net income (loss) per share as their effect would be anti-dilutive. The following potentially dilutive common stock equivalents outstanding at the end of the period were excluded from the computations of weighted-average diluted common shares for the periods indicated as their effects would be anti-dilutive (in thousands): Three Months Ended March 31, 2019 2018 Stock options issued and outstanding 5,340 1,582 Unvested restricted stock units outstanding 40 — Employee stock purchase plan 20 — 5,400 1,582

Supplemental Disclosures

Supplemental Disclosures	3 Months Ended
	Mar. 31, 2019
Payables And Accruals [Abstract]
Supplemental Disclosures	12. Supplemental Disclosures Accrued expenses and other current liabilities consists of the following (in thousands): March 31, 2019 December 31, 2018 Accrued external research and development expenses $ 5,474 $ 4,274 Accrued personnel costs 5,357 9,484 Accrued income taxes payable 1,134 726 Accrued licensing costs 911 1,617 Accrued external general and administrative expenses 854 773 Accrued purchases of property and equipment 191 221 Other accrued expenses and current liabilities 268 69 $ 14,189 $ 17,164 Other liabilities of $1.8 million and $2.5 million reported as of March 31, 2019 and December 31, 2018, respectively, consist of accrued licensing costs payable in periods beyond 12 months from the reporting date.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	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, 2018 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018, which was filed with the SEC on February 27, 2019. Certain information and footnote disclosures required by GAAP which are normally included in the Company’s annual consolidated financial statements have been omitted pursuant to SEC rules and regulations for interim reporting. In the opinion of management, the accompanying consolidated financial statements reflect all adjustments, which include all normal and recurring adjustments necessary for the fair statement of the Company’s financial position as of March 31, 2019, and the results of its operations and its cash flows for the interim periods ended March 31, 2019 and 2018. 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, 2018, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018. 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	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: revenue, stock-based compensation expense, accrued research and development expenses and other accrued liabilities, income taxes and the fair value of financial instruments.
Restricted Cash	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, 2019 March 31, 2018 Cash and cash equivalents $ 55,852 $ 71,870 Restricted cash 1,053 225 Total cash and cash equivalents and restricted cash $ 56,905 $ 72,095
Marketable Securities	Marketable Securities Marketable securities consist of debt securities and are classified as available-for-sale and carried at fair value. Marketable 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. Unrealized gains and losses, 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 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. 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. A decline in the fair value below cost of available-for-sale securities that is deemed other-than-temporary is charged to results of operations, resulting in the establishment of a new cost basis for the security. The Company regularly evaluates whether declines in the fair value of its investments below their cost are other-than-temporary. The evaluation includes consideration of the cause of the impairment, including the creditworthiness of the security issuers, the number of securities in an unrealized loss position, the severity and duration of the unrealized losses, whether the Company has the intent to sell the securities and whether it is more likely than not that the Company will be required to sell the securities before the recovery of their amortized cost basis. The Company has not recorded any impairment of available-for-sale securities which was deemed to be to be other-than-temporary.
Leases	Leases Effective January 1, 2019, the Company adopted Accounting Standards Update (ASU) 2016-02, Leases Leases Under Topic 842, the Company applies a dual approach to all leases in which it is a lessee and classifies leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the Company. Lease classification is evaluated at the inception of the lease agreement. Regardless of classification, the Company records a right-of-use asset and a lease liability for all leases with a term greater than 12 months. All of the Company’s leases as of March 31, 2019 have been classified as operating leases. Operating lease expense is recognized on a straight-line basis over the term of the lease. Variable lease expense is recognized as incurred. The Company identifies leases in its contracts if the contract conveys the right to control the use of identified property, plant or equipment for a period of time in exchange for consideration. The Company does not allocate lease consideration between lease and nonlease components and records a lease liability equal to the present value of the remaining fixed consideration under the lease. The interest rates implicit in the Company’s leases are generally not readily determinable. Accordingly, the Company uses its estimated incremental borrowing rate at the commencement date of the lease to determine the present value discount of the lease liability. The Company estimates its incremental borrowing rate for each lease based on an evaluation of its expected credit rating and the prevailing market rates for collateralized debt in a similar economic environment with similar payment terms and maturity dates commensurate with the term of the lease. The right-of-use asset for each lease is equal to the lease liability, adjusted for unamortized initial direct costs and lease incentives and prepaid or accrued rent. Initial direct costs of entering into a lease are included in the right-of-use asset and amortized as lease expense over the term of the lease. Lease incentives, such as tenant improvements allowances, are recorded as a reduction of the right-of-use asset and amortized as a reduction of lease expense over the term of the lease. The Company excludes options to extend or terminate leases from the calculation of the lease liability unless it is reasonably certain the option will be exercised.
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. ASC 820, Fair Value Measurements and Disclosures • 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 Income (Loss) Per Share	Net Income (Loss) Per Share Basic net income (loss) per share is calculated by dividing net income (loss) applicable to common stockholders by the weighted-average common shares outstanding during the period, without consideration for common stock equivalents. Diluted net income (loss) per share is calculated by adjusting the weighted-average common shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury-stock method. Contingently convertible shares in which conversion is based on non-market-priced contingencies are excluded from the calculations of both basic and diluted net income (loss) per share until the contingency has been fully met. For purposes of the diluted net income (loss) per share calculation, common stock equivalents are excluded from the calculation of diluted net income (loss) per share if their effect would be anti-dilutive.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Adoption of ASU 2016-02, Leases In February 2016, the Financial Accounting Standards Board (FASB) issued ASU 2016-02, Leases Leases The Company elected certain practical expedients allowed by Topic 842 for transition purposes, including the package of practical expedients which permitted the Company to not reassess lease identification, classification and initial direct costs under Topic 842 for leases that commenced prior to January 1, 2019. Additionally, the Company elected the practical expedient allowed for transition purposes to use hindsight in determining the terms of leases that commenced prior to January 1, 2019. Upon the adoption of Topic 842, the Company recorded operating lease right-of-use assets of $7.4 million and operating lease liabilities of $8.4 million for its leases which were in effect and had commenced prior to January 1, 2019 and had original lease terms of more than 12 months. Additionally, upon the adoption of Topic 842, the Company derecognized $5.9 million of property and equipment and $5.9 million of financing lease obligations related to construction-in-progress at 9800 Medical Center Drive, as the Company does not control the building during the construction period under the requirements of Topic 842. The lease term for the facility at 9800 Medical Center Drive does not commence until certain construction is completed by the landlord and the building is delivered to the Company. The right-of-use assets and lease liabilities related to the facility at 9800 Medical Center Drive will not be recognized on the Company’s consolidated balance sheets until the commencement date of the lease, which is expected to occur in 2020. The cumulative impact of the adoption of Topic 842 resulted in an increase in accumulated deficit of less than $0.1 million on January 1, 2019. The adoption of Topic 842 did not have a material impact on the Company’s results of operations for the three months ended March 31, 2019, nor does the Company believe it will have a material impact on future results of operations based on its current leasing arrangements. Other recently adopted accounting pronouncements In February 2018, the FASB issued ASU 2018-02, Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income Tax Cuts and Jobs Act of 2017 (the TCJA) In April 2017, the FASB issued ASU 2017-08, Receivables—Nonrefundable Fees and Other Costs (Subtopic 310-20) Recent accounting pronouncements not yet adopted In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Summary of Reconciliation of Cash and Cash Equivalents and Restricted Cash	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, 2019 March 31, 2018 Cash and cash equivalents $ 55,852 $ 71,870 Restricted cash 1,053 225 Total cash and cash equivalents and restricted cash $ 56,905 $ 72,095

Marketable Securities (Tables)

Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2019
Investments Debt And Equity Securities [Abstract]
Summary of Company Marketable Securities	The following tables present a summary of the Company’s marketable securities, which consist solely of available-for-sale debt securities (in thousands): Amortized Cost Unrealized Gains Unrealized Losses Fair Value March 31, 2019 U.S. government and federal agency securities $ 105,533 $ 202 $ (3 ) $ 105,732 Certificates of deposit 9,237 42 — 9,279 Corporate bonds 272,988 502 (45 ) 273,445 $ 387,758 $ 746 $ (48 ) $ 388,456 Amortized Cost Unrealized Gains Unrealized Losses Fair Value December 31, 2018 U.S. government and federal agency securities $ 103,410 $ 93 $ (37 ) $ 103,466 Certificates of deposit 8,992 — — 8,992 Corporate bonds 282,902 36 (377 ) 282,561 $ 395,304 $ 129 $ (414 ) $ 395,019
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	The following tables present the fair values and unrealized losses of marketable securities held by the Company in an unrealized loss position for less than 12 months and 12 months or greater (in thousands): Less than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses March 31, 2019 U.S. government and federal agency securities $ 13,923 $ (3 ) $ — $ — $ 13,923 $ (3 ) Corporate bonds 110,103 (44 ) 1,999 (1 ) 112,102 (45 ) $ 124,026 $ (47 ) $ 1,999 $ (1 ) $ 126,025 $ (48 ) Less than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses December 31, 2018 U.S. government and federal agency securities $ 53,124 $ (37 ) $ — $ — $ 53,124 $ (37 ) Corporate bonds 245,283 (354 ) 12,424 (23 ) 257,707 (377 ) $ 298,407 $ (391 ) $ 12,424 $ (23 ) $ 310,831 $ (414 )

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2019
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, 2019 Cash equivalents: Money market mutual funds $ — $ 55,834 $ — $ 55,834 Total cash equivalents — 55,834 — 55,834 Marketable securities: U.S. government and federal agency securities — 105,732 — 105,732 Certificates of deposit — 9,279 — 9,279 Corporate bonds — 273,445 — 273,445 Total marketable securities — 388,456 — 388,456 Total cash equivalents and marketable securities $ — $ 444,290 $ — $ 444,290 Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total December 31, 2018 Cash equivalents: Money market mutual funds $ — $ 75,542 $ — $ 75,542 Total cash equivalents — 75,542 — 75,542 Marketable securities: U.S. government and federal agency securities — 103,466 — 103,466 Certificates of deposit — 8,992 — 8,992 Corporate bonds — 282,561 — 282,561 Total marketable securities — 395,019 — 395,019 Total cash equivalents and marketable securities $ — $ 470,561 $ — $ 470,561

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	3 Months Ended
	Mar. 31, 2019
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment, net consists of the following (in thousands): March 31, 2019 December 31, 2018 Lab equipment $ 15,563 $ 14,417 Computer equipment and software 2,104 2,002 Furniture and fixtures 1,935 1,915 Leasehold improvements 12,371 11,751 Construction-in-progress — 5,854 Total property and equipment 31,973 35,939 Accumulated depreciation and amortization (8,833 ) (7,237 ) Property and equipment, net $ 23,140 $ 28,702

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Summary of Lease Costs and Supplemental Cash Flow Information of Operating Leases	The following table summarizes the Company’s lease costs and supplemental cash flow information related to operating leases (in thousands): Three Months Ended March 31, 2019 Operating lease cost $ 700 Variable lease cost 161 Total lease cost $ 861 Cash paid for amounts included in the measurement of operating lease liabilities $ 711 Right-of-use assets acquired through operating lease liabilities $ 36
Weighted-Average Remaining Lease Term and Weighted-Average Discount Rate of Operating Leases	The weighted-average remaining lease term and weighted-average discount rate of the Company’s operating leases were as follows: As of March 31, 2019 Weighted-average remaining lease term (years) 3.1 Weighted-average discount rate 5.6 %
Reconciliation of Undiscounted Future Minimum Lease Payments Remaining Under 9800 Medical Center Drive Lease and Other Operating Leases	The following table presents a reconciliation of the undiscounted future minimum lease payments remaining under the 9800 Medical Center Drive Lease and other operating leases to the amounts reported as operating lease liabilities on the consolidated balance sheet as of March 31, 2019 (in thousands): 9800 Medical Other Total Center Drive Operating Undiscounted Lease (a) Leases Fixed Payments Undiscounted future minimum lease payments: 2019 (remainder of year) $ — $ 1,995 $ 1,995 2020 — 3,088 3,088 2021 1,332 2,412 3,744 2022 4,308 623 4,931 2023 5,188 479 5,667 Thereafter 76,892 — 76,892 Total undiscounted future minimum lease payments $ 87,720 $ 8,597 $ 96,317 Amount representing imputed interest (717 ) Total operating lease liabilities 7,880 Current portion of operating lease liabilities (2,397 ) Operating lease liabilities, non-current $ 5,483 (a) Undiscounted future minimum lease payments under the 9800 Medical Center Drive Lease are not included in the lease liabilities reported on the consolidated balance sheet as of March 31, 2019 as the lease has not yet commenced. The actual timing and amounts of payments under the 9800 Medical Center Drive Lease are subject to adjustment based on the completion date of construction and actual square footage of the facility constructed. Accordingly, these amounts were estimates as of March 31, 2019.
Schedule of Future Minimum Lease Payments Under Topic 840 for 9800 Medical Center Drive Lease and Other Operating Leases	As of December 31, 2018, future minimum lease payments under Topic 840 for the 9800 Medical Center Drive Lease and other operating leases were as follows (in thousands): 9800 Medical Other Total Center Drive Operating Minimum Lease Lease (a) Leases Payments 2019 $ — $ 2,798 $ 2,798 2020 — 3,054 3,054 2021 1,329 2,391 3,720 2022 4,289 621 4,910 2023 5,156 479 5,635 Thereafter 76,420 — 76,420 Total minimum lease payments $ 87,194 $ 9,343 $ 96,537 (a) Includes all future minimum lease payments under the 9800 Medical Center Drive Lease, including amounts recorded as financing lease obligations on the consolidated balance sheet . The actual timing and amounts of payments under the 9800 Medical Center Drive Lease are subject to adjustment based on the completion date of construction and actual square footage of the facility constructed. Accordingly, these amounts were estimates as of December 31, 2018.

License Revenue (Tables)

License Revenue (Tables)	3 Months Ended
	Mar. 31, 2019
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 Net Additions Balance at of Period (Deductions) End of Period Three Months Ended March 31, 2019 Receivables and contract assets: Accounts receivable, current and non-current $ 31,599 $ (469 ) $ 31,130 Contract assets $ 750 $ 250 $ 1,000 Contract liabilities: Deferred revenue, current and non-current $ 3,933 $ — $ 3,933 Balance at Beginning Net Additions Balance at of Period (Deductions) End of Period Three Months Ended March 31, 2018 Receivables and contract assets: Accounts receivable, current and non-current $ 5,850 $ 52,762 $ 58,612 Contract assets $ 350 $ (100 ) $ 250 Contract liabilities: Deferred revenue, current and non-current $ — $ — $ —

Stock-based Compensation (Table

Stock-based Compensation (Tables)	3 Months Ended
	Mar. 31, 2019
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, 2019 2018 Stock options $ 5,452 $ 3,122 Restricted stock units 68 68 Employee stock purchase plan 198 101 $ 5,718 $ 3,291
Stock-Based Compensation Expense	The Company has recorded aggregate stock-based compensation expense in the consolidated statements of operations and comprehensive income (loss) as follows (in thousands): Three Months Ended March 31, 2019 2018 Research and development $ 2,347 $ 1,530 General and administrative 3,371 1,761 $ 5,718 $ 3,291
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, 2018 40 $ 20.90 Granted — $ — Vested — $ — Forfeited — $ — Unvested balance at March 31, 2019 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, 2018 4,855 $ 19.31 7.6 $ 118,185 Granted 1,102 $ 48.22 Exercised (480 ) $ 7.81 Cancelled or forfeited (137 ) $ 30.72 Outstanding at March 31, 2019 5,340 $ 26.02 7.9 $ 170,518 Exercisable at March 31, 2019 2,491 $ 12.22 6.8 $ 112,324 Vested and expected to vest at March 31, 2019 5,340 $ 26.02 7.9 $ 170,518 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at the dates reported.

Net Income (Loss) Per Share (Ta

Net Income (Loss) Per Share (Tables)	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Computations of Basic and Diluted Net Income (Loss) Per Share	The computations of basic and diluted net income (loss) per share are as follows (in thousands, except per share data): Three Months Ended March 31, 2019 2018 Basic net income (loss) per share: Net income (loss) applicable to common stockholders $ (32,228 ) $ 104,239 Shares used in computation: Weighted-average common shares outstanding 36,366 31,632 Basic net income (loss) per share $ (0.89 ) $ 3.30 Diluted net income (loss) per share: Net income (loss) applicable to common stockholders $ (32,228 ) $ 104,239 Shares used in computation: Weighted-average common shares outstanding 36,366 31,632 Stock options — 2,620 Restricted stock units — 22 Employee stock purchase plan — 1 Weighted-average diluted common shares 36,366 34,275 Diluted net income (loss) per share $ (0.89 ) $ 3.04
Schedules for Computation of Diluted Weighted-Average Shares Outstanding	The following potentially dilutive common stock equivalents outstanding at the end of the period were excluded from the computations of weighted-average diluted common shares for the periods indicated as their effects would be anti-dilutive (in thousands): Three Months Ended March 31, 2019 2018 Stock options issued and outstanding 5,340 1,582 Unvested restricted stock units outstanding 40 — Employee stock purchase plan 20 — 5,400 1,582

Supplemental Disclosures (Table

Supplemental Disclosures (Tables)	3 Months Ended
	Mar. 31, 2019
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, 2019 December 31, 2018 Accrued external research and development expenses $ 5,474 $ 4,274 Accrued personnel costs 5,357 9,484 Accrued income taxes payable 1,134 726 Accrued licensing costs 911 1,617 Accrued external general and administrative expenses 854 773 Accrued purchases of property and equipment 191 221 Other accrued expenses and current liabilities 268 69 $ 14,189 $ 17,164

Nature of Business - Additional

Nature of Business - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accumulated deficit	$ (115,317)	$ (83,016)
Cash, cash equivalents and marketable securities	$ 444,300

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Summary of Reconciliation of Cash and Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Accounting Policies [Abstract]
Cash and cash equivalents	$ 55,852	$ 75,561	$ 71,870
Restricted cash	1,053	$ 1,053	225
Total cash and cash equivalents and restricted cash	$ 56,905		$ 72,095

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	Jan. 01, 2019	Mar. 31, 2019	Dec. 31, 2018
Significant Accounting Policies [Line Items]
Impairment of available-for-sale securities deemed to be other-than-temporary		$ 0
Operating lease right-of-use assets		6,858,000
Property and equipment, net		23,140,000	$ 28,702,000
Financing lease obligation			5,854,000
Accumulated deficit		$ (115,317,000)	$ (83,016,000)
ASU 2016-02 [Member]
Significant Accounting Policies [Line Items]
Operating lease right-of-use assets	$ 7,400,000
Operating lease liabilities	8,400,000
ASU 2016-02 [Member] | Maximum [Member]
Significant Accounting Policies [Line Items]
Accumulated deficit	100,000
ASU 2016-02 [Member] | Restatement Adjustment [Member] | 9800 Medical Center Drive Lease [Member]
Significant Accounting Policies [Line Items]
Property and equipment, net	(5,900,000)
Financing lease obligation	(5,900,000)
ASU 2018-02 [Member] | Maximum [Member]
Significant Accounting Policies [Line Items]
Reclassification of stranded tax effects of unrealized gains and losses on available-for-sale securities from accumulated other comprehensive loss to accumulated deficit	$ 100,000

Marketable Securities - Summary

Marketable Securities - Summary of Company Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 387,758	$ 395,304
Unrealized Gains	746	129
Unrealized Losses	(48)	(414)
Fair Value	388,456	395,019
U.S. Government and Federal Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	105,533	103,410
Unrealized Gains	202	93
Unrealized Losses	(3)	(37)
Fair Value	105,732	103,466
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	9,237	8,992
Unrealized Gains	42
Fair Value	9,279	8,992
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	272,988	282,902
Unrealized Gains	502	36
Unrealized Losses	(45)	(377)
Fair Value	$ 273,445	$ 282,561

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2019USD ($)Security	Mar. 31, 2018USD ($)	Dec. 31, 2018USD ($)
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities remaining maturities greater than three years	$ 0		$ 0
Unrealized gains (losses) on available-for-sale securities, before tax	1,000,000	$ (200,000)
Unrealized gains (losses) on available-for-sale securities, income tax expense	$ 400,000	$ 0
Number of investment grade fixed income security | Security	40
Other-than-temporary impaired	$ 0		$ 0
Investment Income [Member] | Maximum [Member]
Schedule Of Available For Sale Securities [Line Items]
Net realized gains or losses available for sale securities	$ 100,000

Marketable Securities - Summa_2

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, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 124,026
Less than 12 Months, Unrealized Losses	(47)	$ (391)
12 Months or Greater, Fair Value	1,999
12 Months or Greater, Unrealized Losses	(1)	(23)
Total, Fair Value	126,025
Total, Unrealized Losses	(48)	(414)
Less than 12 Months, Fair Value		298,407
12 Months or Greater, Fair Value		12,424
Total, Fair Value		310,831
U.S. Government and Federal Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	13,923
Less than 12 Months, Unrealized Losses	(3)	(37)
Total, Fair Value	13,923
Total, Unrealized Losses	(3)	(37)
Less than 12 Months, Fair Value		53,124
Total, Fair Value		53,124
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	110,103
Less than 12 Months, Unrealized Losses	(44)	(354)
12 Months or Greater, Fair Value	1,999
12 Months or Greater, Unrealized Losses	(1)	(23)
Total, Fair Value	112,102
Total, Unrealized Losses	$ (45)	(377)
Less than 12 Months, Fair Value		245,283
12 Months or Greater, Fair Value		12,424
Total, Fair Value		$ 257,707

Fair Value of Financial Instr_3

Fair Value of Financial Instruments - Schedule of Fair Value of Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Marketable securities:
Fair Value	$ 388,456	$ 395,019
Cash Equivalents and Marketable Securities [Member]
Marketable securities:
Total cash equivalents and marketable securities	444,290	470,561
Corporate Bonds [Member]
Marketable securities:
Fair Value	273,445	282,561
Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	55,834	75,542
U.S. Government and Federal Agency Securities [Member]
Marketable securities:
Fair Value	105,732	103,466
Certificates of Deposit [Member]
Marketable securities:
Fair Value	9,279	8,992
Money Market Mutual Funds [Member] | Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	55,834	75,542
Significant Other Observable Inputs (Level 2) [Member]
Marketable securities:
Fair Value	388,456	395,019
Significant Other Observable Inputs (Level 2) [Member] | Cash Equivalents and Marketable Securities [Member]
Marketable securities:
Total cash equivalents and marketable securities	444,290	470,561
Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member]
Marketable securities:
Fair Value	273,445	282,561
Significant Other Observable Inputs (Level 2) [Member] | Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	55,834	75,542
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Federal Agency Securities [Member]
Marketable securities:
Fair Value	105,732	103,466
Significant Other Observable Inputs (Level 2) [Member] | Certificates of Deposit [Member]
Marketable securities:
Fair Value	9,279	8,992
Significant Other Observable Inputs (Level 2) [Member] | Money Market Mutual Funds [Member] | Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	$ 55,834	$ 75,542

Fair Value of Financial Instr_4

Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
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
Non-marketable equity securities	400,000		$ 400,000
Remeasurements or impairment losses on non-marketable equity securities	$ 0	$ 0

Property and Equipment Net - Sc

Property and Equipment Net - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Total property and equipment	$ 31,973	$ 35,939
Accumulated depreciation and amortization	(8,833)	(7,237)
Property and equipment, net	23,140	28,702
Lab Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	15,563	14,417
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	2,104	2,002
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,935	1,915
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	$ 12,371	11,751
Construction in Progress [Member]
Property Plant And Equipment [Line Items]
Total property and equipment		$ 5,854

Property and Equipment Net - Ad

Property and Equipment Net - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, net	$ 23,140		$ 28,702
ASU 2016-02 [Member] | Restatement Adjustment [Member] | 9800 Medical Center Drive Lease [Member]
Property Plant And Equipment [Line Items]
Property and equipment, net		$ (5,900)

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended						3 Months Ended
	Apr. 30, 2019USD ($)ft²	Nov. 30, 2018USD ($)ft²	Apr. 30, 2018USD ($)	Nov. 30, 2017USD ($)	Jan. 31, 2016	Mar. 31, 2015	Mar. 31, 2019USD ($)	Jan. 01, 2019USD ($)	Dec. 31, 2018USD ($)
Other Commitments [Line Items]
Property and equipment, net							$ 23,140,000		$ 28,702,000
Financing lease obligation									5,854,000
European Patent Office Proceeding [Member]
Other Commitments [Line Items]
Liability related to third party opposition							$ 0		$ 0
Subsequent Event [Member] | University of Pennsylvania [Member] | Maximum [Member]
Other Commitments [Line Items]
Upfront fee, milestone fees	$ 20,500,000
Rockville, Maryland and New York, New York [Member]
Other Commitments [Line Items]
Operating leases expiration year							2022
New York [Member]
Other Commitments [Line Items]
Letters of credit							$ 200,000
9800 Medical Center Drive Lease [Member] | Subsequent Event [Member]
Other Commitments [Line Items]
Payment to landlord	$ 4,000,000
9800 Medical Center Drive Lease [Member] | Leasehold Improvements [Member]
Other Commitments [Line Items]
Property and equipment, net							$ 300,000
9800 Medical Center Drive Lease [Member] | ASU 2016-02 [Member] | Restatement Adjustment [Member]
Other Commitments [Line Items]
Property and equipment, net								$ (5,900,000)
Financing lease obligation								$ (5,900,000)
9800 Medical Center Drive Lease [Member] | Rockville, Maryland [Member]
Other Commitments [Line Items]
Number of square feet to be constructed | ft²		132,000
Construction completion, description							Construction of the new building, which will be conducted by the landlord, is expected to be completed in 2020
Lease term, from the delivery of leased premises		16 years
Tenant improvement allowance entitled to received		$ 14,600,000
Lease not yet commenced, option to extend the lease							true
Lease not yet commenced, additional lease term		10 years
Lease not yet commenced, option to terminate the lease							true
Lease not yet commenced, option to terminate the lease, description							the option to terminate the lease after 12 years from the delivery of the leased premises to the Company. If the Company elects to terminate the lease, it will be subject to a termination fee equal to the unamortized tenant improvement allowance, rent abatement and landlord commissions as of the termination date, bearing interest at 5% per annum, plus four months of base rent and operating expenses.
Minimum release term for option to terminate		12 years
Percentage of early termination fee		5.00%
Term of required base rent to be paid as termination fee		4 months
Frequency of periodic payment							Monthly payments under the 9800 Medical Center Drive Lease begin approximately 12 months from the delivery of the leased premises to the Company and escalate annually in accordance with the lease agreement.
Periodic payment term, from the delivery of leased premises		12 months
Letters of credit		$ 800,000
9800 Medical Center Drive Lease [Member] | Rockville, Maryland [Member] | Subsequent Event [Member]
Other Commitments [Line Items]
Number of square feet to be constructed | ft²	5,975
Change in tenant improvement allowance	$ 15,300,000
Increased on future rent	$ 4,000,000
9712 Medical Center Drive [Member] | Rockville, Maryland [Member]
Other Commitments [Line Items]
Lease term commenced date						2015-04
Lease term							In September 2015, November 2015, July 2017 and April 2018, the Company amended the 9712 Medical Center Drive Lease to include additional office and laboratory space at 9714 Medical Center Drive, and ultimately extend the term of the lease to September 2021.
Operating lease, existense of option to extend the lease							true
Additional lease term under option to extend			6 years
Tenant improvement allowance received			$ 400,000
9600 Blackwell Road [Member] | Rockville, Maryland [Member]
Other Commitments [Line Items]
Percentage of early termination fee					8.00%
Lease term commenced date					2016-02
Operating lease, existense of option to extend the lease							true
Additional lease term under option to extend					5 years
Tenant improvement allowance received				$ 800,000
Lease period					7 years 6 months
Lease expiration date					Sep. 30, 2023
Monthly lease payments commencement period					2016-09
Required period to terminate lease after commencement period					67 months

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Lease Costs and Supplemental Cash Flow Information of Operating Leases (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)
Leases [Abstract]
Operating lease cost	$ 700
Variable lease cost	161
Total lease cost	861
Cash paid for amounts included in the measurement of operating lease liabilities	711
Right-of-use assets acquired through operating lease liabilities	$ 36

Commitments and Contingencies_3

Commitments and Contingencies - Weighted-Average Remaining Lease Term and Weighted-Average Discount Rate of Operating Leases (Detail)	Mar. 31, 2019
Leases [Abstract]
Weighted-average remaining lease term (years)	3 years 1 month 6 days
Weighted-average discount rate	5.60%

Commitments and Contingencies_4

Commitments and Contingencies - Reconciliation of Undiscounted Future Minimum Lease Payments Remaining Under 9800 Medical Center Drive Lease and Other Operating Leases (Detail) $ in Thousands	Mar. 31, 2019USD ($)
Undiscounted future minimum lease payments:
2019 (remainder of year)	$ 1,995
2020	3,088
2021	3,744
2022	4,931
2023	5,667
Thereafter	76,892
Total undiscounted future minimum lease payments	96,317
Current portion of operating lease liabilities	(2,397)
Operating lease liabilities, non-current	5,483
9800 Medical Center Drive Lease [Member]
Undiscounted future minimum lease payments:
2021	1,332
2022	4,308
2023	5,188
Thereafter	76,892
Total undiscounted future minimum lease payments	87,720
Other Operating leases [Member]
Undiscounted future minimum lease payments:
2019 (remainder of year)	1,995
2020	3,088
2021	2,412
2022	623
2023	479
Total undiscounted future minimum lease payments	8,597
Amount representing imputed interest	(717)
Total operating lease liabilities	7,880
Current portion of operating lease liabilities	(2,397)
Operating lease liabilities, non-current	$ 5,483

Commitments and Contingencies_5

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Topic 840 for 9800 Medical Center Drive Lease and Other Operating Leases (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Lessee Lease Description [Line Items]
2019	$ 2,798
2020	3,054
2021	3,720
2022	4,910
2023	5,635
Thereafter	76,420
Total minimum lease payments	96,537
9800 Medical Center Drive Lease [Member]
Lessee Lease Description [Line Items]
2021	1,329
2022	4,289
2023	5,156
Thereafter	76,420
Total minimum lease payments	87,194
Other Operating leases [Member]
Lessee Lease Description [Line Items]
2019	2,798
2020	3,054
2021	2,391
2022	621
2023	479
Total minimum lease payments	$ 9,343

License Revenue - Additional In

License Revenue - Additional Information (Detail)	1 Months Ended			3 Months Ended		12 Months Ended
	May 31, 2018USD ($)	Jan. 31, 2018USD ($)	Mar. 31, 2014USD ($)	Mar. 31, 2019USD ($)ProductCandidateTechnologyLicensee	Mar. 31, 2018USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
License Revenue [Line Items]
Milestone payment upon commencement of clinical trials in humans				$ 29,600,000
Milestone payment upon submission of regulatory approval filings				47,500,000
Milestone payment upon approval of commercial products by regulatory agencies				117,500,000
Milestone payment upon achievement of specified sales targets for licensed products				232,000,000
Deferred revenue, current and non-current				3,933,000		$ 3,933,000
Impirment losses on receivables or contract assets				0		0
Total current and non-current accounts receivable				31,130,000	$ 58,612,000	31,599,000	$ 5,850,000
Billed to customers				100,000		400,000
Billable to customers in future periods				$ 31,000,000		31,200,000
Transaction price description				Upon its execution, the transaction price of the January 2018 Amendment was $132.1 million, which was fully recognized as license revenue upon the delivery of the modified license in January 2018. In May 2018, as a result of the acquisition of AveXis by Novartis, the transaction price was increased by $40.0 million to account for the acceleration of the sale-based milestone which was previously excluded from the transaction price. The $40.0 million increase in the transaction price was recognized as license revenue upon the completion of the change of control in May 2018 since the amended license had been fully delivered to AveXis. Additionally, due to the acceleration of the two $30.0 million payments originally due in January 2019 and January 2020, the Company recognized $6.1 million of interest income from licensing upon the completion of the change of control of AveXis, which represents the remaining present value discount on such payments as of the date of the change of control. As of March 31, 2019, the transaction price of the January 2018 Amendment was $172.1 million, which includes: (i) the $80.0 million payment in January 2018, (ii) the present value, as of the date of the January 2018 Amendment, of the two $30.0 million payments originally due in January 2019 and January 2020 and (iii) the $40.0 million sales-based milestone which was accelerated upon the change of control in May 2018. Variable consideration under the January 2018 Amendment, which has been excluded from the transaction price, includes the remaining sales-based milestone payment of $80.0 million, as well as any potential sublicense fees or royalties on sales of licensed products, which will be recognized in the period of the underlying sales or sublicenses, if any.
Interest income from licensing				$ 613,000	1,355,000
Revenue				884,000	132,391,000
Accounts receivable, current				$ 8,372,000		8,587,000
NAV Technology Licensees [Member] | NAV Technology Platform [Member]
License Revenue [Line Items]
Number of technology licensees | TechnologyLicensee				11
NAV Technology Licensees [Member] | NAV Technology Platform [Member] | Minimum [Member]
License Revenue [Line Items]
Number of development partnered product candidates | ProductCandidate				20
January 2018 Amendment Agreement [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
Accounts receivable					53,500,000
License agreement period		14 years
License fee		$ 80,000,000
Sales-based milestone payment unpaid in the event of change of control		40,000,000
License fees received	$ 100,000,000
Transaction price of license		132,100,000		$ 172,100,000
Interest income from licensing	6,100,000
Sales-based milestone payment exclude transaction price				80,000,000
Payments due from related party				$ 30,000,000
Consideration payment due period one				2019-01
Consideration payment due period two				2020-01
January 2018 Amendment Agreement [Member] | AveXis, Inc. [Member] | Share-based Milestone [Member]
License Revenue [Line Items]
Increase in transaction price of license	40,000,000
January 2018 Amendment Agreement [Member] | AveXis, Inc. [Member] | First Anniversary [Member]
License Revenue [Line Items]
License fee		30,000,000
January 2018 Amendment Agreement [Member] | AveXis, Inc. [Member] | Second Anniversary [Member]
License Revenue [Line Items]
License fee		30,000,000
January 2018 Amendment Agreement [Member] | Minimum [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
Milestone payment		$ 120,000,000
January 2018 Amendment Agreement [Member] | License [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
Increase in transaction price of license	$ 40,000,000
License Agreement [Member] | License [Member]
License Revenue [Line Items]
License revenue				$ 800,000	300,000
November 2018 License Agreement [Member] | Abeona Therapeutics Inc. [Member]
License Revenue [Line Items]
Accounts receivable				26,700,000		26,000,000
March 2014 License Agreement [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
Accounts receivable				200,000		200,000
Up-front fee paid			$ 2,000,000	2,000,000
Milestone fee payments upon achievement of various development and commercialization			$ 12,300,000
License agreement amendment date			2018-01
Transaction price of license				7,500,000
Payments for remaining development milestones				7,000,000
Interest income from licensing					1,200,000
Accounts receivable, non -current				200,000		200,000
March 2014 License Agreement [Member] | Maximum [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
Interest income from licensing				100,000
Accounts receivable, current				100,000		$ 100,000
March 2014 License Agreement [Member] | License [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
Revenue				$ 0	$ 132,100,000

License Revenue - Summary of Ch

License Revenue - Summary of Changes in Balances of Receivables, Contract Assets and Contract Liabilities (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Receivables and contract assets:
Accounts receivable, current and non-current, Balance at Beginning of Period	$ 31,599	$ 5,850
Accounts receivable, current and non-current, Net Additions (Deductions)	(469)	52,762
Accounts receivable, current and non-current, Balance at End of Period	31,130	58,612
Contract assets, Balance at Beginning of Period	750	350
Contract assets, Net Additions (Deductions)	250	(100)
Contract assets, Balance at End of Period	1,000	$ 250
Contract liabilities:
Deferred revenue, current and non-current, Balance at Beginning of Period	3,933
Deferred revenue, current and non-current, Balance at End of Period	$ 3,933

License Revenue - Additional _2

License Revenue - Additional Information (Detail1)	Mar. 31, 2019
January 2018 Amendment Agreement [Member] | AveXis, Inc. [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2018-01-01
License Revenue [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, period	1 year

Stock-based Compensation - Addi

Stock-based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
	Jan. 31, 2019	Mar. 31, 2019	Mar. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense		$ 68,900
Unrecognized stock-based compensation, weighted-average period		3 years 1 month 6 days
Proceeds from exercise of stock options		$ 3,762	$ 3,824
2015 Equity Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional shares to be issued	1,444,808
Common stock shares authorized for issuance		10,933,221
Shares available for future grants		2,583,144
Weighted-average fair values of options granted		$ 32.20
Exercise of stock options, Shares		480,320
Total intrinsic value of options exercised		$ 20,000
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		159,339
Common stock shares issued to participants		10,241

Stock-based Compensation - Stoc

Stock-based Compensation - Stock-Based Compensation Expense by Award Type (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 5,718	$ 3,291
Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	5,452	3,122
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	68	68
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 198	$ 101

Stock-based Compensation - St_2

Stock-based Compensation - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 5,718	$ 3,291
Research and Development Costs [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	2,347	1,530
General and Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 3,371	$ 1,761

Stock-based Compensation - Summ

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, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]
Shares Outstanding, Beginning Balance	4,855
Shares, Granted	1,102
Shares, Exercised	(480)
Shares, Cancelled or forfeited	(137)
Shares Outstanding, Ending Balance	5,340	4,855
Shares, Exercisable	2,491
Shares, Vested and expected to vest	5,340
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-average Exercise Price Outstanding, Beginning Balance	$ 19.31
Weighted-average Exercise Price, Granted	48.22
Weighted-average Exercise Price, Exercised	7.81
Weighted-average Exercise Price, Cancelled or forfeited	30.72
Weighted-average Exercise Price, Outstanding, Ending Balance	26.02	$ 19.31
Weighted-average Exercise Price, Exercisable	12.22
Weighted-average Exercise Price, Vested and expected to vest	$ 26.02
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average Remaining Contractual Life (Years) Outstanding	7 years 10 months 24 days	7 years 7 months 6 days
Weighted-average Remaining Contractual Life (Years), Exercisable	6 years 9 months 18 days
Weighted-average Remaining Contractual Life (Years), Vested and expected to vest	7 years 10 months 24 days
Aggregate Intrinsic Value Outstanding	$ 170,518	$ 118,185
Aggregate Intrinsic Value, Exercisable	112,324
Aggregate Intrinsic Value, Vested and expected to vest	$ 170,518

Stock-based Compensation - Su_2

Stock-based Compensation - Summary of Unvested RSUs Activity Under 2015 Plan (Detail) - Restricted Stock Units [Member] shares in Thousands	3 Months Ended
	Mar. 31, 2019$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Unvested, Beginning Balance	40
Shares, Granted	0
Shares, Vested	0
Shares, Forfeited	0
Shares Unvested, Ending Balance	40
Weighted-average Grant Date Fair Value, Unvested Beginning Balance | $ / shares	$ 20.90
Weighted-average Grant Date Fair Value, Unvested Ending Balance | $ / shares	$ 20.90

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Income Tax Disclosure [Line Items]
Income tax benefit	$ 70
Unrealized gains (losses) on available-for-sale securities, income tax expense	400	$ 0
Accrued income tax provision	1,134		$ 726
Accrued Expenses and Other Current Liabilities [Member]
Income Tax Disclosure [Line Items]
Accrued income tax provision	$ 300

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
FOXKISER,LLP [Member] | Service Agreements [Member] | Research and Development Costs [Member]
Related Party Transaction [Line Items]
Related party transaction expense	$ 0.8	$ 0.5

Net Income (Loss) Per Share - C

Net Income (Loss) Per Share - Computations of Basic and Diluted Net Income (Loss) Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Basic net income (loss) per share:
Net income (loss) applicable to common stockholders	$ (32,228)	$ 104,239
Shares used in computation:
Weighted-average common shares outstanding	36,366	31,632
Basic net income (loss) per share	$ (0.89)	$ 3.30
Diluted net income (loss) per share:
Net income (loss) applicable to common stockholders	$ (32,228)	$ 104,239
Shares used in computation:
Weighted-average common shares outstanding	36,366	31,632
Weighted-average diluted common shares	36,366	34,275
Diluted net income (loss) per share	$ (0.89)	$ 3.04
Stock Option [Member]
Shares used in computation:
Weighted-average diluted common shares		2,620
Restricted Stock Units [Member]
Shares used in computation:
Weighted-average diluted common shares		22
Employee Stock Purchase Plan [Member]
Shares used in computation:
Weighted-average diluted common shares		1

Net Income (Loss) Per Share - S

Net Income (Loss) Per Share - Schedules for Computation of Diluted Weighted-Average Shares Outstanding (Detail) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	5,400	1,582
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	5,340	1,582
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	20

Supplemental Disclosures - Sche

Supplemental Disclosures - Schedules of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued external research and development expenses	$ 5,474	$ 4,274
Accrued personnel costs	5,357	9,484
Accrued income taxes payable	1,134	726
Accrued licensing costs	911	1,617
Accrued external general and administrative expenses	854	773
Accrued purchases of property and equipment	191	221
Other accrued expenses and current liabilities	268	69
Accrued expenses and other current liabilities	$ 14,189	$ 17,164

Supplemental Disclosures - Addi

Supplemental Disclosures - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Other liabilities	$ 1,772	$ 2,505