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Fair Value Measurements (Details) (USD $)
Mar. 31, 2014
Dec. 31, 2013
Fair Value Measurements    
Fair value derivative instruments Level 3 to Level 2 Transfers Amount   $ 54,400,000
Fair value on a recurring basis | Fair Value
   
Assets    
Derivative instruments, at fair value 79,821,000 107,647,000
Liabilities    
Derivative instruments, at fair value 59,333,000 29,651,000
Fair value on a recurring basis | Quoted prices in active markets for identical assets (Level 1)
   
Assets    
Derivative instruments, at fair value 246,000 240,000
Liabilities    
Derivative instruments, at fair value   315,000
Fair value on a recurring basis | Significant other observable inputs (Level 2)
   
Assets    
Derivative instruments, at fair value 79,575,000 107,407,000
Liabilities    
Derivative instruments, at fair value $ 59,333,000 $ 29,336,000