Long-Term Debt Senior Convertible Notes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Senior Convertible Notes, net carrying amount	$ 143,430		$ 143,430		$ 139,107
1.50% Senior Convertible Notes Due 2021 [Member]
Debt Instrument [Line Items]
Senior Convertible Notes, face amount	$ 172,500		$ 172,500		172,500
Senior Convertible Notes, interest rate, stated percentage	1.50%		1.50%
Senior Convertible Notes, interest expense	$ 2,600	$ 2,500	$ 5,200	$ 4,900
Senior Convertible Notes, unamortized debt discount	(26,319)		(26,319)		(30,183)
Unamortized deferred financing costs	(2,751)		(2,751)		(3,210)
Senior Convertible Notes, net carrying amount	$ 143,430		$ 143,430		$ 139,107