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Derivatives (Details 1) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2011
Dec. 31, 2010
Fair value of the Company's commodity derivatives in the balance sheet    
Forward commodity contracts included in Commodity derivative assets - current $ 37,560 $ 226,216
Forward commodity contracts included in Commodity derivative assets - noncurrent 2,289 18,113
Forward commodity contracts included in Commodity derivative liabilities - current (15,874) (57,621)
Forward commodity contracts included in Commodity derivative liabilities - noncurrent (1,519) (3,279)
Regulated futures and options contracts included in Commodity derivatives (23,367) (105,030)
Over-the-counter contracts included in Commodity derivatives 2,887 (41,300)
Total estimated fair value of commodity derivatives $ 1,976 $ 37,099