Senior Convertible Notes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Senior Convertibles Notes [Line Items]
Senior Convertible Notes, net of unamortized discount and deferred financing costs	$ 152,503		$ 152,503		$ 147,894
1.50% Senior Convertible Notes Due 2021 [Member]
Senior Convertibles Notes [Line Items]
Senior Convertible Notes, principal amount	$ 172,500		$ 172,500		172,500
Senior Convertible Notes, interest rate, stated percentage	1.50%		1.50%
Senior Convertible Notes, interest expense	$ 2,700	$ 2,600	$ 5,400	$ 5,200
Senior Convertible Notes, unamortized debt discount	(18,162)		(18,162)		(22,313)
Senior Convertible Notes, unamortized deferred financing costs	(1,835)		(1,835)		(2,293)
Senior Convertible Notes, net of unamortized discount and deferred financing costs	$ 152,503		$ 152,503		$ 147,894