0.25% Convertible Senior Notes, Convertible Note Hedge and Warrant - Schedule of Net Equity Impact, Included in Additional Paid-in Capital, of the Liability and Equity Components (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Total	$ 73,713
0.25% Convertible Senior Notes Due 2022 as Net Equity Impact [Member]
Debt Instrument [Line Items]
Conversion Option	106,006
Purchase of Convertible Note Hedges	(78,920)
Sales of warrants	59,080
Issuance costs	(3,054)
Deferred tax liability	(9,399)
Total	$ 73,713