Debt - Convertible Senior Notes - Interest and Amortization Expense (Details) - Convertible senior notes - 2025 Notes - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Interest expense
Contractual interest expense	$ 1,203	$ 1,203	$ 2,406	$ 2,393
Amortization of debt issuance costs	136	125	269	248
Amortization of debt discount	1,635	1,496	3,236	2,975
Total interest and amortization expense	$ 2,974	$ 2,824	$ 5,911	$ 5,616

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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