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Fair Value Measurements (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2012
Dec. 31, 2011
Mar. 31, 2012
Minimum
Mar. 31, 2012
Maximum
Mar. 31, 2012
Fair value on a recurring basis
Fair Value
Dec. 31, 2011
Fair value on a recurring basis
Fair Value
Mar. 31, 2012
Fair value on a recurring basis
Quoted prices in active markets for identical assets (Level 1)
Dec. 31, 2011
Fair value on a recurring basis
Quoted prices in active markets for identical assets (Level 1)
Mar. 31, 2012
Fair value on a recurring basis
Significant other observable inputs (Level 2)
Dec. 31, 2011
Fair value on a recurring basis
Significant other observable inputs (Level 2)
Mar. 31, 2012
Fair value on a recurring basis
Significant unobservable inputs (Level 3)
Dec. 31, 2011
Fair value on a recurring basis
Significant unobservable inputs (Level 3)
Fair value assets and liabilities measured on a recurring basis                        
Range of derived market volatilities (as a percent)     25.00% 48.00%                
Assets                        
Derivative instruments, at fair value $ 608,647 $ 512,161     $ 608,647 $ 512,161 $ 2,077 $ 3,612 $ 450,504 $ 365,238 $ 156,066 $ 143,311
Total assets         608,647 512,161 2,077 3,612 450,504 365,238 156,066 143,311
Liabilities                        
Derivative instruments, at fair value         120,044 123,306 2,774 2,727 117,270 120,528   51
Total liabilities         $ 120,044 $ 123,306 $ 2,774 $ 2,727 $ 117,270 $ 120,528   $ 51