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Fair Value Measurements (Details) (USD $)
In Thousands
Jun. 30, 2011
Dec. 31, 2010
Assets    
Derivative instruments, at fair value $ 211,986 $ 225,339
Total assets 7,293,089 7,098,438
Liabilities    
Derivative instruments, at fair value 97,531 106,721
Total liabilities 4,056,534 4,019,742
Fair Value | Recurring Basis
   
Assets    
Derivative instruments, at fair value 211,986  
Total assets 211,986  
Liabilities    
Derivative instruments, at fair value 97,531  
Total liabilities 97,531  
Quoted prices in active markets for identical assets (Level 1) | Recurring Basis
   
Assets    
Derivative instruments, at fair value 2,698  
Total assets 2,698  
Liabilities    
Derivative instruments, at fair value 3,339  
Total liabilities 3,339  
Significant other observable inputs (Level 2) | Recurring Basis
   
Assets    
Derivative instruments, at fair value 107,316  
Total assets 107,316  
Liabilities    
Derivative instruments, at fair value 94,192  
Total liabilities 94,192  
Significant unobservable inputs (Level 3) | Recurring Basis
   
Assets    
Derivative instruments, at fair value 101,972  
Total assets $ 101,972