License and Royalty Revenue - Additional Information (Detail)	1 Months Ended			3 Months Ended		6 Months Ended
	May 31, 2018 USD ($)	Jan. 31, 2018 USD ($)	Mar. 31, 2014 USD ($)	Jun. 30, 2019 USD ($) ProductCandidate TechnologyLicensee	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) ProductCandidate TechnologyLicensee	Jun. 30, 2018 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
License Revenue [Line Items]
Milestone payment upon commencement of clinical trials in humans						$ 18,600,000
Milestone payment upon submission of regulatory approval filings						32,500,000
Milestone payment upon approval of commercial products by regulatory agencies						81,000,000.0
Milestone payment upon achievement of specified sales targets for licensed products						187,000,000.0
Accounts receivable				$ 33,634,000	$ 5,224,000	33,634,000	$ 5,224,000	$ 31,130,000	$ 31,599,000	$ 58,621,000	$ 5,850,000
Deferred revenue, current and non-current				3,333,000	600,000	3,333,000	600,000	3,933,000	3,933,000
Licenses revenue				7,881,000	40,031,000	8,765,000	172,422,000
Impirment losses on receivables or contract assets						0
Billed to customers				200,000		200,000			400,000
Billable to customers in future periods				33,400,000		33,400,000			31,200,000
Allowance for doubtful accounts				0		$ 0			0
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 June 30, 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.
Interest income from licensing				762,000	6,898,000	$ 1,375,000	8,253,000
Accounts receivable, current				9,679,000		9,679,000			8,587,000
AveXis, Inc. [Member]
License Revenue [Line Items]
Interest income from licensing				7,000	6,752,000	15,000	7,950,000
License [Member]
License Revenue [Line Items]
Licenses revenue				600,000	0	600,000
License [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
Licenses revenue				$ 4,424,000	40,000,000	$ 4,424,000	172,066,000
NAV Technology Licensees [Member] | NAV Technology Platform [Member]
License Revenue [Line Items]
Number of technology licensees | TechnologyLicensee				9		9
NAV Technology Licensees [Member] | NAV Technology Platform [Member] | Minimum [Member]
License Revenue [Line Items]
Number of development partnered product candidates | ProductCandidate				20		20
March 2014 Licence Agreement [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
License fees received					100,000,000.0
Accounts receivable										$ 53,300,000
Accounts receivable				$ 1,100,000		$ 1,100,000			200,000
Up-front fee paid			$ 2,000,000.0			2,000,000.0
Milestone fee payments upon achievement of various development and commercialization			$ 12,300,000
License agreement amendment date			2018-01
Transaction price of license				11,000,000.0		11,000,000.0
Payments for remaining development milestones				3,500,000		3,500,000
Increase in transaction price of license				3,500,000		3,500,000
Accounts receivable, current				900,000		900,000
Accounts receivable, non -current				200,000		200,000
March 2014 Licence Agreement [Member] | Maximum [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
Accounts receivable, current									100,000
Accounts receivable, non -current									200,000
License Agreement [Member] | License [Member]
License Revenue [Line Items]
License revenue				3,200,000	$ 39,900,000	4,000,000.0	$ 200,000
Proceeds from milestone license revenue	$ 40,000,000.0
November 2018 License Agreement [Member] | Abeona Therapeutics Inc. [Member]
License Revenue [Line Items]
Accounts receivable				27,300,000		27,300,000			$ 26,000,000.0
January 2018 Amendment Agreement [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
License fees received	100,000,000.0
License agreement period		14 years
License fee		$ 80,000,000.0
Sales-based milestone payment unpaid in the event of change of control		40,000,000.0
Transaction price of license		132,100,000		$ 172,100,000		172,100,000
Increase in transaction price of license						0
Interest income from licensing	6,100,000
Sales-based milestone payment exclude transaction price						$ 80,000,000.0
Payments due from related party								$ 30,000,000.0
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	40,000,000.0
January 2018 Amendment Agreement [Member] | AveXis, Inc. [Member] | First Anniversary [Member]
License Revenue [Line Items]
License fee		30,000,000.0
January 2018 Amendment Agreement [Member] | AveXis, Inc. [Member] | Second Anniversary [Member]
License Revenue [Line Items]
License fee		30,000,000.0
January 2018 Amendment Agreement [Member] | Minimum [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
Milestone payment		$ 120,000,000.0
January 2018 Amendment Agreement [Member] | License [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
Increase in transaction price of license	$ 40,000,000.0