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Convertible Senior Notes (Details) (USD $)
 6 Months Ended 12 Months Ended
Jun. 30, 2013
Dec. 31, 2012
May 31, 2012
Put Rights [Member]
Jun. 30, 2013
3.625% Convertible Senior Notes [Member]
Dec. 31, 2012
3.625% Convertible Senior Notes [Member]
May 31, 2012
3.625% Convertible Senior Notes [Member]
May 31, 2005
3.625% Convertible Senior Notes [Member]
Jun. 30, 2013
5.875% Convertible Senior Notes [Member]
Dec. 31, 2012
5.875% Convertible Senior Notes [Member]
Nov. 30, 2005
5.875% Convertible Senior Notes [Member]
May 31, 2005
5.875% Convertible Senior Notes [Member]
Debt Instrument [Line Items]                      
Aggregate principal amount       $ 450,000 $ 450,000   $ 250,000,000.0 $ 139,467,000 $ 139,467,000 $ 300,000,000.0  
Interest rate on notes (in hundredths)           3.625% 3.625%     5.875% 5.875%
Repayment due to exercise of existing put rights     83,500,000                
Components of consolidated balance sheets associated with convertible senior notes [Abstract]                      
Face Amount       450,000 450,000   250,000,000.0 139,467,000 139,467,000 300,000,000.0  
Discount (44,289,000) (44,582,000)                  
Net carrying value 95,628,000 95,335,000                  
Carrying amount of equity component included in additional paid-in capital 108,714,000 108,714,000                  
Excess of instruments' if-converted values over face principal amounts $ 0 $ 0