Long-Term Debt (Details 1) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
2.25% Convertible Senior Notes due 2036 [Member]
Carrying value of Notes related discount and equity component
Carrying amount of equity component	$ 65,270	$ 65,270
Allocated underwriter fees, net of taxes	(1,475)	(1,475)
Allocated debt issuance cost, net of taxes	(58)	(58)
Total net equity component	63,737	63,737
Deferred income tax component	7,281	8,023
Principal amount of Notes	182,753	182,753
Unamortized discount	(19,562)	(21,574)
Unamortized underwriter fees	(766)	(845)
Secured Long-term Debt, Noncurrent	162,425	160,334
Unamortized debt issuance cost	30	33
3.00% Convertible Senior Notes due 2020 [Member]
Carrying value of Notes related discount and equity component
Carrying amount of equity component	25,359	25,359
Allocated underwriter fees, net of taxes	(760)	(760)
Allocated debt issuance cost, net of taxes	(112)	(112)
Total net equity component	24,487	24,487
Deferred income tax component	10,302	10,625
Principal amount of Notes	115,000	115,000
Unamortized discount	(28,194)	(29,094)
Unamortized underwriter fees	(1,551)	(1,601)
Secured Long-term Debt, Noncurrent	85,255	84,305
Unamortized debt issuance cost	$ 229	$ 236