Debt - Carrying Value - Convertible Senior Notes (Details) - Convertible Notes - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Jan. 10, 2017
Debt
Principal amount	$ 345,000	$ 345,000	$ 345,000
Unamortized debt discount	(52,336)	(63,022)	$ (72,500)
Unamortized transaction costs	(5,272)	(6,348)
Net carrying value	$ 287,392	$ 275,630

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

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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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