Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Product revenue related reserves	$ 29,054	$ 26,146
Accrued contract manufacturing costs	16,369	15,794
Accrued employee compensation costs	14,763	24,692
Accrued clinical and pre-clinical costs	12,041	11,396
Accrued professional fees	8,000	11,319
Accrued BioMarin royalties	4,455	8,254
Accrued interest expense	3,183	1,045
Accrued property and equipment	2,878	5,421
Accrued collaboration cost sharing	2,458	2,167
Accrued research costs	1,815	1,070
Accrued milestone expense	1,517	24,020
Other	3,587	2,771
Total accrued expenses	$ 100,120	$ 134,095