Carrying Value of Convertible Senior Notes (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Less: Current portion	$ (102,400)
Total convertible senior notes, long term		$ 99,250
2.125% Convertible Senior Notes due 2021
Debt Instrument [Line Items]
Principal amount	114,989	115,000
Unamortized debt discount	(10,706)	(13,395)
Unamortized debt issuance costs	(1,883)	(2,355)
Total convertible senior notes	102,400	99,250
Less: Current portion	$ (102,400)
Total convertible senior notes, long term		$ 99,250

Carrying value as of the balance sheet date of long-term debt (with maturities initially due after one year or beyond the operating cycle if longer) identified as Convertible Notes Payable, excluding current portion. Convertible Notes Payable is a written promise to pay a note which can be exchanged for a specified amount of another, related security, at the option of the issuer and the holder.

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Carrying value as of the balance sheet date of the portion of long-term debt due within one year or the operating cycle if longer identified as Convertible Notes Payable. Convertible Notes Payable is a written promise to pay a note which can be exchanged for a specified amount of another, related security, at the option of the issuer and the holder.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Amount, after accumulated amortization, of debt discount.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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