Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued employee compensation costs	$ 17,949	$ 14,402
Product revenue related reserves	16,867	7,302
Accrued clinical and preclinical costs	11,068	15,975
Accrued contract manufacturing costs	8,421	14,019
Accrued professional fees	7,578	6,794
Accrued BioMarin royalties	3,695	2,846
Accrued collaboration cost sharing	2,699
Accrued interest expense	1,251	1,291
Accrued research costs	729	401
Accrued income taxes	523	943
Accrued property and equipment	289	2,525
Other	1,408	2,484
Total accrued expenses	$ 72,477	$ 68,982