Convertible Senior Notes, Note Hedges, and Warrants - Schedule of Convertible Notes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Net carrying amount	$ 318,957	$ 304,484
Liability Component Gross Value [Member]
Debt Instrument [Line Items]
Liability component	345,000	345,000
Liability Component [Member]
Debt Instrument [Line Items]
Debt discount, net of amortization	(23,178)	(36,115)
Debt issuance costs, net of amortization	(2,865)	(4,401)
Net carrying amount	318,957	304,484
Equity Component Gross Value [Member]
Debt Instrument [Line Items]
Net carrying amount	63,643	63,643
Equity Component [Member]
Debt Instrument [Line Items]
Debt issuance costs, net of amortization	(1,582)	(1,582)
Net carrying amount	$ 62,061	$ 62,061