Debt - Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Debt Instrument [Line Items]
Total amortization of debt issuance costs and accretion of equity portion	$ 1,507	$ 0
Total interest expense related to the Notes	2,136	$ 0
1.75% Convertible Senior Notes due 2023
Debt Instrument [Line Items]
Amortization of debt discount	1,329
Amortization of debt issuance costs	178
Total amortization of debt issuance costs and accretion of equity portion	1,507
Contractual interest expense	629
Total interest expense related to the Notes	$ 2,136
Effective interest rate of the liability component	7.25%