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Financial Instruments (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis    
Derivative assets $ 527 $ 1,347
Derivative liabilities (2,730) (1,425)
Fair Value, Measurements, Recurring [Member]
    
Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis    
Derivative assets 527 1,347
Derivative liabilities (2,730) (1,425)
Commodity derivative contracts [Member] | Fair Value, Measurements, Recurring [Member]
    
Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis    
Derivative assets 527 1,347
Derivative liabilities (2,633) (1,371)
Interest rate derivative contracts [Member] | Fair Value, Measurements, Recurring [Member]
    
Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis    
Derivative liabilities (97) (54)
Quoted Prices in Active Markets For Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
    
Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis    
Derivative assets      
Derivative liabilities      
Quoted Prices in Active Markets For Identical Assets (Level 1) [Member] | Commodity derivative contracts [Member] | Fair Value, Measurements, Recurring [Member]
    
Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis    
Derivative assets      
Derivative liabilities      
Quoted Prices in Active Markets For Identical Assets (Level 1) [Member] | Interest rate derivative contracts [Member] | Fair Value, Measurements, Recurring [Member]
    
Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis    
Derivative liabilities      
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
    
Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis    
Derivative assets      
Derivative liabilities      
Significant Other Observable Inputs (Level 2) [Member] | Commodity derivative contracts [Member] | Fair Value, Measurements, Recurring [Member]
    
Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis    
Derivative assets      
Derivative liabilities      
Significant Other Observable Inputs (Level 2) [Member] | Interest rate derivative contracts [Member] | Fair Value, Measurements, Recurring [Member]
    
Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis    
Derivative liabilities      
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
    
Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis    
Derivative assets 527 1,347
Derivative liabilities (2,730) (1,425)
Significant Unobservable Inputs (Level 3) [Member] | Commodity derivative contracts [Member] | Fair Value, Measurements, Recurring [Member]
    
Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis    
Derivative assets 527 1,347
Derivative liabilities (2,633) (1,371)
Significant Unobservable Inputs (Level 3) [Member] | Interest rate derivative contracts [Member] | Fair Value, Measurements, Recurring [Member]
    
Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis    
Derivative liabilities $ (97) $ (54)