Debt - Convertible Senior Notes - Interest and Amortization Expense (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Interest expense recognized
Contractual interest expense	$ 1,203	$ 1,203	$ 3,609	$ 3,609
Amortization of debt issuance costs	166	152	487	446
Amortization of debt discount	1,997	1,828	5,859	5,363
Total interest and amortization expense	$ 3,366	$ 3,183	$ 9,955	$ 9,418

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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Represents the portion of interest incurred in the period on debt arrangements that was charged against earnings, excluding amortization of debt discount (premium) and financing costs.

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