Notes Payable - 2.25% Convertible Senior Notes Due 2022 - Total Interest Expense (Details) - 2.25% Convertible Senior Notes due 2022 - Convertible Senior Notes - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Interest Expense
Contractual interest expense	$ 1,802	$ 1,889	$ 3,609	$ 3,777
Amortization of debt issuance costs	360	283	705	555
Amortization of debt discount	5,156	4,164	10,224	8,239
Total interest expense	$ 7,318	$ 6,336	$ 14,538	$ 12,571

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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