Summary of Other Income and Loss (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Nonoperating Income Expense [Abstract]
Interest expenses									$ (5,781)	$ (1,892)	$ (1,365)
Interest income									1,809	507	890
Other income									1,982	850	629
Gain from sale of Priority Review Voucher				$ 125,000					125,000
Total other income (loss)	$ (2,693)	$ 184	$ 184	$ 125,335	$ (57)	$ (209)	$ (201)	$ (68)	$ 123,010	$ (535)	$ 154