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	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Interest Expense
Contractual interest expense	$ 1,888	$ 1,888	$ 3,777	$ 3,777
Amortization of debt issuance costs	237	196	464	383
Amortization of debt discount	3,816	3,497	7,550	6,918
Total interest expense	$ 5,941	$ 5,581	$ 11,791	$ 11,078

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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