Convertible Senior Notes - Schedule of Convertible Notes (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2014
Debt Instrument [Line Items]
Net carrying amount	$ 326,512	$ 318,957
Liability component, gross
Debt Instrument [Line Items]
Liability component	345,000	345,000
Liability component
Debt Instrument [Line Items]
Debt discount, net of amortization	(16,423)	(23,178)
Debt issuance costs, net of amortization	(2,065)	(2,865)
Net carrying amount	326,512	318,957
Equity component, gross
Debt Instrument [Line Items]
Net carrying amount	63,643	63,643	$ 63,600
Equity component
Debt Instrument [Line Items]
Debt issuance costs, net of amortization	(1,582)	(1,582)
Net carrying amount	$ 62,061	$ 62,061