Convertible Senior Notes - Schedule of Interest Expense (Details) - 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]
Amortization of debt issuance costs			$ 545	$ 346
Amortization of debt discount			5,230	3,089
Convertible Senior Notes Due February 2023 \| Convertible Debt
Debt Instrument [Line Items]
Contractual interest expense	$ 431	$ 431	868	647
Amortization of debt issuance costs	250	223	500	346
Amortization of debt discount	2,342	1,990	4,640	3,089
Total	3,023	2,644	6,008	4,082
Convertible Senior Notes Due June 2026 \| Convertible Debt
Debt Instrument [Line Items]
Contractual interest expense	138	0	138	0
Amortization of debt issuance costs	45	0	45	0
Amortization of debt discount	590	0	590	0
Total	$ 773	$ 0	$ 773	$ 0

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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