Liabilities - Convertible Senior Notes (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
	Jan. 10, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Long-term debt
Proceeds received, net of transaction fees					$ 335,751
Convertible Senior 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		(57,779)		(57,779)		(63,022)
Unamortized transaction costs		(5,820)		(5,820)		(6,348)
Net carrying value		281,401		281,401		$ 275,630
Cash Interest Expense
Coupon interest expense		2,329	$ 2,329	4,658	4,244
Non-Cash Interest Expense
Amortization of debt discount		2,646	2,455	5,243	4,440
Amortization of transaction costs		266	247	528	447
Total Interest Expense		5,241	$ 5,031	10,429	$ 9,131
Convertible Senior Notes | Level 2
Non-Cash Interest Expense
Estimated fair value of convertible senior notes		$ 304,300		$ 304,300