Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued employee compensation costs	$ 21,915	$ 14,402
Product revenue related reserves	21,684	7,302
Accrued clinical and pre-clinical costs	13,778	15,975
Accrued professional fees	9,112	6,794
Accrued contract manufacturing costs	8,348	14,019
Accrued BioMarin royalties	4,002	2,846
Accrued interest expense	3,183	1,291
Accrued property and equipment	2,515	2,525
Accrued research costs	2,506	401
Accrued collaboration cost sharing	1,200
Accrued income taxes		943
Other	444	2,484
Total accrued expenses	$ 88,687	$ 68,982