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Derivatives (Details 2) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Effect of Derivative Instruments on the Consolidated Balance Sheets    
Fair value of derivative assets, current $ 7,495 $ 0
Fair value of derivative assets, noncurrent 4,236 0
Asset Derivatives, Fair Value 11,731 0
Fair value of derivative liabilities, current 0 5,633
Fair value of derivative liabilities, noncurrent 0 494
Liability Derivatives, Fair Value 0 6,127
Assets    
Fair value of derivatives-gross presentation 17,851 26
Effects of netting arrangements (6,120) (26)
Asset Derivatives, Fair Value 11,731 0
Liabilities    
Fair value of derivatives-gross presentation 6,120 6,153
Effects of netting arrangements (6,120) (26)
Liability Derivatives, Fair Value 0 6,127
Nondesignated
   
Effect of Derivative Instruments on the Consolidated Balance Sheets    
Fair value of derivative assets, current 7,495 0
Fair value of derivative assets, noncurrent 4,236 0
Fair value of derivative liabilities, current 0 5,633
Fair value of derivative liabilities, noncurrent 0 494
Commodity Derivatives | Nondesignated
   
Effect of Derivative Instruments on the Consolidated Balance Sheets    
Asset Derivatives, Fair Value 11,731 0
Liability Derivatives, Fair Value 0 6,127
Assets    
Asset Derivatives, Fair Value 11,731 0
Liabilities    
Liability Derivatives, Fair Value $ 0 $ 6,127