Debt - Components of Convertible Senior Notes (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2014
Debt Instrument [Line Items]
Less: current portion	$ 0	$ (10,184)
Long-term debt	0	227,394
Convertible Senior Notes \| 3.25% Convertible Senior Notes due 2020
Debt Instrument [Line Items]
Principal amount outstanding	58,506	167,314
Unamortized discount of liability component	(4,075)	(15,356)
Unamortized debt issuance costs	(546)	(2,090)	$ (4,200)
Net carrying amount of liability component	53,885	149,868
Less: current portion	(53,885)	0
Long-term debt	0	149,868
Carrying value of equity component, net of issuance costs	10,093	29,211
Fair value of outstanding Convertible Senior Notes	$ 118,767	$ 257,245
Remaining amortization period of discount on the liability component	2 years 3 months	3 years	6 years

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 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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