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Supplemental Financial Information Exploration and Production (Details 5) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2011
Mcfe
Dec. 31, 2010
Mcfe
Dec. 31, 2009
Mcfe
Dec. 31, 2008
Mcfe
Reserve Quantities [Line Items]        
Proved Developed Reserves, Net 476,763,000 494,426,000 482,329,000  
Proved Developed and Undeveloped Reserves, Extensions, Discoveries, and Additions 65,049,000 56,523,000 42,297,000  
Proved Developed and Undeveloped Reserves [Abstract]        
Proved Developed Reserves, Net 476,763,000 494,426,000 482,329,000  
Proved Undeveloped Reserves, Net 109,145,000 151,170,000 171,395,000  
Proved Developed and Undeveloped Reserves, Net 585,908,000 645,596,000 653,724,000 810,371,000
Change in PUD reserve (42,000,000)      
Amount of PUD reserves converted into proved developed reserves 27,100,000      
Drilling and completion capital $ 62.9      
Negative revision to PUD reserves 53,600,000      
New PUD reserves 38,700,000      
Future development costs estimated to be spent to develop PUD reserves year one 109.3      
Future development costs estimated to be spent to develop PUD reserves year two 47.8      
Future development costs estimated to be spent to develop PUD reserves year three $ 13.7      
Natural Gas Reserves [Member]
        
Reserve Quantities [Line Items]        
Proved Developed Reserves, Net 303,495 334,911 321,561  
Proved Developed and Undeveloped Reserves, Extensions, Discoveries, and Additions 28,221 36,191 26,882  
Proved Developed and Undeveloped Reserves [Abstract]        
Proved Developed Reserves, Net 303,495 334,911 321,561  
Proved Undeveloped Reserves, Net 76,332 113,486 126,864  
Proved Developed and Undeveloped Reserves, Net 379,827 448,397 448,425 604,282
Oil Reserves [Member]
        
Reserve Quantities [Line Items]        
Proved Developed Reserves, Net 28,878 26,586 26,794  
Proved Developed and Undeveloped Reserves, Extensions, Discoveries, and Additions 6,138 3,389 2,569  
Proved Developed and Undeveloped Reserves [Abstract]        
Proved Developed Reserves, Net 28,878 26,586 26,794  
Proved Undeveloped Reserves, Net 5,469 6,281 7,422  
Proved Developed and Undeveloped Reserves, Net 34,347 32,867 34,216 34,348