Litigation Settlement and License Agreements - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended	9 Months Ended
	Jul. 31, 2017	Sep. 30, 2017	Sep. 30, 2017	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront fee to be paid on execution of agreements	$ 35,000,000
Intangible asset		$ 14,029,000	$ 14,029,000	$ 8,076,000
Settlement and license charges		25,588,000	28,427,000
U.S. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Intangible asset		6,600,000	6,600,000
BioMarin Pharmaceuticals, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front cash payment under agreements	$ 35,000,000		35,000,000
Amortization of intangible assets and royalties license agreement execution expenses		$ 800,000	$ 2,300,000
BioMarin Pharmaceuticals, Inc. [Member] | U.S. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Royalty payment expiry period	2023-12
BioMarin Pharmaceuticals, Inc. [Member] | EU [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Royalty payment expiry period	2024-09
Settlement Agreement [Member] | BioMarin Pharmaceuticals, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront fee to be paid on execution of agreements	$ 20,000,000
License Agreement [Member] | BioMarin and Academisch Ziekenhuis Leiden [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront fee to be paid on execution of agreements	15,000,000
Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Regulatory and sales milestone payments	$ 65,000,000
Percentage of royalty payments	8.00%
Minimum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of royalty payments	4.00%