Long-Term Debt Senior Convertible Notes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Senior Convertible Notes, net carrying amount	$ 145,662		$ 145,662		$ 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	$ 7,800	$ 7,400
Senior Convertible Notes, unamortized debt discount	(24,316)		(24,316)		(30,183)
Unamortized deferred financing costs	(2,522)		(2,522)		(3,210)
Senior Convertible Notes, net carrying amount	$ 145,662		$ 145,662		$ 139,107