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Related Party Transactions - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Related Party Transaction [Line Items]
Accounts receivable from related party	$ 19,434		$ 19,434		$ 4,004
Accrued liabilities to related party	353		353		272
Astellas [Member] \| Collaborative Arrangement [Member]
Related Party Transaction [Line Items]
Revenue related to collaboration agreements	5,100	$ 5,300	30,800	$ 15,100
Expense related to collaboration agreements	400	$ 200	1,100	$ 800
Accounts receivable from related party	19,400		19,400		4,000
Accrued liabilities to related party	400		400		$ 300
Astellas [Member] \| Astellas Agreement [Member] \| API Shipment [Member]
Related Party Transaction [Line Items]
Accrued liabilities to related party	20,900		20,900
Unbilled Revenues [Member] \| Astellas [Member] \| Regulatory Milestone [Member]
Related Party Transaction [Line Items]
Revenue related to collaboration agreements	$ 14,900		$ 15,000