Debt - Carrying amount of notes and interest expense (Details) - 1.375% Convertible Senior Notes - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Mar. 06, 2017
Debt
Total Debt issuance costs			$ 10,600
Liability component
Principal	$ 400,000	$ 400,000	$ 400,000
Unamortized debt discount	(53,247)	(53,247)
Unamortized debt issuance costs	(8,036)	(8,036)
Net carrying amount	338,717	338,717
Equity component
Net carrying amount	$ 57,735	$ 57,735
Effective interest rate	4.75%	4.75%
Amortization of debt discount and debt issuance costs		4 years 4 months 24 days
Interest expense related to the Notes
Contractual interest expense	$ 1,360	$ 3,117
Amortization of debt discount	2,674	6,068
Amortization of debt issuance costs	404	916
Interest Expense, Total	$ 4,438	$ 10,101