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Notes Payable (Liability and Equity Components of Convertible Senior Notes) (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended 3 Months Ended 12 Months Ended 3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Mar. 31, 2014
September 2015 Notes [Member]
Dec. 31, 2013
September 2015 Notes [Member]
Mar. 31, 2014
December 2029 Notes [Member]
Dec. 31, 2013
December 2029 Notes [Member]
Mar. 31, 2014
Convertible Debt [Member]
Mar. 31, 2013
Convertible Debt [Member]
Debt Instrument [Line Items]                
Principal amount of liability component     $ 91,951 $ 91,951 $ 37,032 $ 68,373    
Unamortized discount     (5,712) (6,623) (1,513) (3,743)    
Net carrying amount of liability component     86,239 85,328 35,519 64,630    
Carrying amount of equity component         4,983 15,586    
Amount by which the if-converted value exceeds principal     83,860 104,403 56,960 113,169    
Interest expense on convertible notes $ 10,264 $ 7,248         $ 3,400 $ 7,200