Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accrued clinical and preclinical costs	$ 15,975	$ 10,033
Accrued employee compensation costs	14,402	8,748
Accrued contract manufacturing costs	14,019	4,673
Product revenue related reserves	7,302	305
Accrued professional fees	6,794	2,799
Accrued BioMarin royalties	2,846
Accrued property and equipment	2,525	1,186
Accrued interest expenses	1,291	100
Accrued income taxes	943
Accrued research costs	401	1,186
Other	2,484	1,986
Total accrued expenses	$ 68,982	$ 31,016