Convertible Notes Payable - Summary of Total Interest Expense Recognized Related to the Convertible Notes (Details) - Convertible debt - 3.00% Convertible notes - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Contractual interest expense	$ 2,288	$ 2,287	$ 6,863	$ 6,863
Amortization of debt discount	3,865	3,379	11,216	9,804
Amortization of debt issuance costs	145	127	421	370
Total interest expense	$ 6,298	$ 5,793	$ 18,500	$ 17,037

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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