Convertible Senior Notes - Schedule of Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Debt Instrument [Line Items]
Amortization of debt issuance costs	$ 250	$ 123
Amortization of debt discount	2,298	$ 1,099
Convertible Senior Notes Due February 2023 | Convertible Debt
Debt Instrument [Line Items]
Contractual interest expense	437
Amortization of debt issuance costs	250
Total	$ 2,985