Convertible Senior Notes - Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Total amortization of debt issuance costs and accretion of equity portion			$ 1,128	$ 0
Total interest expense related to the Notes	$ 1,610	$ 0	$ 1,610	$ 0
1.75% Convertible Senior Notes due 2023
Debt Instrument [Line Items]
Amortization of debt discount	997
Amortization of debt issuance costs	131
Total amortization of debt issuance costs and accretion of equity portion	1,128
Contractual interest expense	482
Total interest expense related to the Notes	$ 1,610
Effective interest rate of the liability component	7.10%		7.10%