Liabilities - Convertible Senior Notes (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Jan. 10, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Long-term debt
Proceeds received, net of transaction fees					$ 335,752
Convertible Notes
Long-term debt
Principal amount	$ 345,000	$ 345,000		$ 345,000		$ 345,000
Interest rate (as a percent)	2.70%
Proceeds received, net of transaction fees	$ 335,800
Unamortized debt discount		(55,082)		(55,082)		(63,022)
Unamortized transaction costs		(5,549)		(5,549)		(6,348)
Net carrying value		284,369		284,369		$ 275,630
Cash Interest Expense
Coupon interest expense		2,329	$ 2,329	6,986	6,573
Non-Cash Interest Expense
Amortization of debt discount		2,697	2,502	7,940	6,942
Amortization of transaction costs		271	252	799	699
Total Interest Expense		5,297	$ 5,083	15,725	$ 14,214
Convertible Notes | Level 2
Non-Cash Interest Expense
Estimated fair value of convertible senior notes		$ 292,000		$ 292,000