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Quarterly Financial Results (Unaudited) Quarterly Financial Results (Unaudited) (Parenthetical) (Details) (USD $)
 3 Months Ended 12 Months Ended
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Quarterly Financial Information [Line Items]                      
Goodwill impairment loss $ 235,396,000 $ 0                  
Asset impairment charges                 316,004,000 2,825,000 0
Unrealized gain (loss) on derivatives                 217,755,000 101,034,000 (283,604,000)
Gas Treating Plants and CO2 Compression Facilities
                      
Quarterly Financial Information [Line Items]                      
Asset impairment charges 79,300,000                    
Dynamic Acquisition
                      
Quarterly Financial Information [Line Items]                      
Business Combination Provisional Information Initial Accounting Incomplete Adjustments Total     4,800,000                
Exploration and Production
                      
Quarterly Financial Information [Line Items]                      
Goodwill impairment loss 235,396,000                    
Commodity Derivatives
                      
Quarterly Financial Information [Line Items]                      
Unrealized gain (loss) on derivatives (19,300,000) (222,500,000) 580,700,000 (129,200,000) (429,100,000) 604,600,000 188,300,000 (269,000,000) 209,701,000 94,788,000 (275,209,000)
Commodity Derivatives | Exploration and Production
                      
Quarterly Financial Information [Line Items]                      
Unrealized gain (loss) on derivatives                 $ 209,701,000 $ 94,788,000 $ (275,209,000)