License Revenue - Additional Information (Detail)	1 Months Ended			3 Months Ended		12 Months Ended
	May 31, 2018 USD ($)	Jan. 31, 2018 USD ($)	Mar. 31, 2014 USD ($)	Mar. 31, 2019 USD ($) ProductCandidate TechnologyLicensee	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
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