Equity and Liability Components of Notes (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Unamortized discount and fees of liability component	$ (6,505)	$ (7,122)
Less: current portion	(37,106)	(46,020)
Long-term debt	503,825	504,091
Convertible Senior Notes
Debt Instrument [Line Items]
Principal amount of the Notes outstanding	35,328	44,561
Unamortized discount and fees of liability component	(103)	(514)
Net carrying amount of liability component	35,225	44,047
Less: current portion	(35,225)	(44,047)
Long-term debt	0	0
Carrying value of equity component, net of issuance costs	$ (15,456)	$ (7,626)
Remaining amortization period of discount on the liability component	1 month 6 days	4 months 24 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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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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