Summary of Other Income and Loss (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Nonoperating Income Expense [Abstract]
Interest expense									$ (33,709)	$ (5,801)	$ (1,892)
Interest income									6,810	1,809	507
Amortization of investment discount (premium)									8,573	1,401	(80)
Other (loss) income									(656)	601	930
Gain from sale of Priority Review Voucher								$ 125,000		125,000
Total other (loss) income	$ (2,311)	$ (6,968)	$ (5,218)	$ (4,485)	$ (2,693)	$ 184	$ 184	$ 125,335	$ (18,982)	$ 123,010	$ (535)