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Convertible Notes (Details 2) (USD $)
 3 Months Ended 12 Months Ended
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2010
Sep. 30, 2010
Jun. 30, 2010
Mar. 31, 2010
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Dec. 31, 2010
Zero Coupon Convertible Senior Notes due 2010
Dec. 31, 2009
Zero Coupon Convertible Senior Notes due 2010
Dec. 31, 2011
5% Convertible Senior Notes due 2014
Dec. 31, 2010
5% Convertible Senior Notes due 2014
Dec. 31, 2009
5% Convertible Senior Notes due 2014
Jun. 29, 2009
5% Convertible Senior Notes due 2014
Interest expense related to notes                                  
2014 Notes coupon interest at a rate of 5%                           $ 8,625,000 $ 8,625,000 $ 4,326,000  
Amortization of discount at an additional effective interest rate                       958,000 10,998,000 12,622,000 10,116,000 5,626,000  
Total interest expense 5,453,000 5,410,000 5,212,000 5,172,000 4,990,000 4,953,000 3,740,000 6,016,000 21,247,000 19,699,000 20,950,000            
Convertible notes stated interest rate (as a percent)                           5.00% 5.00% 5.00% 5.00%
Effective interest rate (as a percent)                       8.40% 8.40% 11.70% 11.70% 11.70%  
Adjustment to interest expense due to incorrect amortization of the non-cash debt discount                 $ 700,000