Debt - Convertible Senior Notes - Interest and Amortization Expense (Details) - Convertible senior notes - 2025 Notes - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Interest expense
Contractual interest expense	$ 1,203	$ 1,190
Amortization of debt issuance costs	133	123
Amortization of debt discount	1,601	1,479
Total interest and amortization expense	$ 2,937	$ 2,792

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

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Amount of amortization expense attributable to debt issuance costs.

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Amount of the cost of borrowed funds accounted for as interest expense for debt.

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Represents the portion of interest incurred in the period on debt arrangements that was charged against earnings, excluding amortization of debt discount (premium) and financing costs.

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