Annual report pursuant to Section 13 and 15(d)

Hedging Activities (Narrative) (Details)

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Hedging Activities (Narrative) (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 12 Months Ended
Sep. 30, 2011
financial_institution
Dec. 31, 2012
financial_institution
Dec. 31, 2011
Jul. 31, 2012
Feb. 29, 2012
Dec. 31, 2010
Feb. 28, 2010
General Discussion of Derivative Instruments and Hedging Activities [Abstract]              
Number of financial institutions 4 4          
Expected to reclassify out of accumulated other comprehensive income in next 12 months   $ 9,660          
Loss on fair value hedge ineffectiveness   144          
Gain on fair value hedge ineffectiveness     $ 25        
Production barrels per day (bbls/day)   4,000 2,000 3,000 2,000 2,300 3,000
Forward contract sale, price per barrel   107.29 86.96 109.73 108.00 58.24 54.81
Delivered percentage of production under fixed price swaps   46.00%