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	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Debt Instrument [Line Items]
Contractual interest expense	$ 2,288	$ 2,293	$ 4,575	$ 4,580
Amortization of debt discount	3,737	3,267	7,351	6,425
Amortization of debt issuance costs	140	123	276	243
Total interest expense	$ 6,165	$ 5,683	$ 12,202	$ 11,248

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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