Convertible Senior Notes - Schedule of Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Amortization of debt issuance costs		$ 587	$ 28
Amortization of debt discount		5,370	$ 0
Convertible Senior Notes Due February 2023 \| Convertible Debt
Debt Instrument [Line Items]
Contractual interest expense	$ 379	1,027
Amortization of debt issuance costs	241	587
Amortization of debt discount	2,282	5,370
Total	$ 2,902	$ 6,984

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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