Notes Payable - 2.25% Convertible Senior Notes due 2022 - Total Interest Expense (Details) - 2.25% Convertible Senior Notes due in 2022 - Convertible Senior Notes - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Interest Expense
Contractual interest expense	$ 1,888	$ 1,888
Amortization of debt issuance costs	226	187
Amortization of debt discount	3,734	3,421
Total interest expense	$ 5,848	$ 5,496

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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