Debt - Interest expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Interest expense
Total interest expense	$ 18,179	$ 4,424	$ 43,101	$ 13,117
Convertible senior notes due October 2021
Interest expense
Contractual interest expense	1,509	1,509	4,528	4,528
Amortization of debt discount	3,139	2,779	9,231	8,172
Amortization of debt issuance costs	125	136	380	416
Total interest expense	$ 4,773	$ 4,424	$ 14,139	$ 13,116