Debt - Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jul. 23, 2018
Debt Instrument [Line Items]
Total amortization of debt issuance costs and accretion of equity portion			$ 4,674	$ 1,128
Total interest expense related to the Notes	$ 2,274	$ 1,610	6,564	1,610
1.75% Convertible Senior Notes due 2023
Debt Instrument [Line Items]
Amortization of debt discount	1,359	997	4,031	997
Amortization of debt issuance costs	182	131	541	131
Total amortization of debt issuance costs and accretion of equity portion	1,541	1,128	4,572	1,128
Contractual interest expense	629	482	1,887	482
Total interest expense related to the Notes	$ 2,170	$ 1,610	$ 6,459	$ 1,610
Effective interest rate of the liability component					7.25%