Equity and Liability Components of Notes (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Principal amount of the Notes outstanding	$ 557,233	$ 239,097
Unamortized discount and fees of liability component	(7,122)	(3,164)
Less: current portion	(46,020)	(2,468)
Long-term debt	504,091	233,465
Convertible Senior Notes
Debt Instrument [Line Items]
Principal amount of the Notes outstanding	44,561	48,951
Unamortized discount and fees of liability component	(514)	(2,183)
Net carrying amount of liability component	44,047	46,768
Less: current portion	(44,047)	0
Long-term debt	0	46,768
Carrying value of equity component, net of issuance costs	$ (7,626)	$ (3,971)
Remaining amortization period of discount on the liability component	4 months 24 days	1 year 3 months 18 days

Including the current and noncurrent portions, carrying value as of the balance sheet date of a written promise to pay a note, initially due after one year or beyond the operating cycle if longer, which can be exchanged for a specified amount of one or more securities (typically common stock), at the option of the issuer or the holder.

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