Liabilities - Convertible Senior Notes (Details) - USD ($) $ in Thousands		3 Months Ended
	Jan. 10, 2017	Mar. 31, 2018	Mar. 31, 2017
Long-term debt
Proceeds received, net of transaction fees			$ 335,752
Convertible Senior Notes
Long-term debt
Principal amount	$ 345,000	$ 345,000	345,000
Interest rate (as a percent)	2.70%
Proceeds received, net of transaction fees	$ 335,800
Unamortized debt discount		(60,424)	(63,022)
Unamortized transaction costs		(6,087)	(6,348)
Net carrying value		278,489	275,630
Cash Interest Expense
Coupon interest expense		2,329	1,915
Non-Cash Interest Expense
Amortization of debt discount		2,598	1,985
Amortization of transaction costs		261	200
Total Interest Expense		5,188	$ 4,100
Convertible Senior Notes | Level 2
Non-Cash Interest Expense
Estimated fair value of Convertible Senior Notes		$ 314,100