Debt - Convertible Senior Notes - Interest and Amortization Expense (Details) - Convertible senior notes - 2025 Notes - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Interest expense
Contractual interest expense	$ 1,203	$ 1,203	$ 3,609	$ 3,596
Amortization of debt issuance costs	139	127	408	375
Amortization of debt discount	1,673	1,532	4,909	4,507
Total interest and amortization expense	$ 3,015	$ 2,862	$ 8,926	$ 8,478

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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