Convertible Notes - Schedule of Debt (Details) - Convertible Notes Payable - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Principal amount of Original Notes	$ 201,250,000	$ 201,250,000
Unamortized debt discount	(5,087,000)	(5,330,000)
Unamortized debt issuance cost	(925,000)	(969,000)
Net carrying value of convertible notes	$ 195,238,000	$ 194,951,000

Face (par) amount of debt instrument at time of issuance.

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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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