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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Contractual interest expense	$ 2,287	$ 1,195	$ 6,863	$ 1,195
Amortization of debt discount	3,379	1,568	9,804	1,568
Amortization of debt issuance costs	127	0	370	0
Total interest expense	$ 5,793	$ 2,763	$ 17,037	$ 2,763

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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