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Fair Value Measurements Fair Value Measurements Using Significant Unobservable Inputs (Details) (USD $)
 0 Months Ended 3 Months Ended
Apr. 16, 2013
Dec. 31, 2012
Dec. 31, 2012
Private Partnerships
Dec. 31, 2011
Private Partnerships
Dec. 31, 2012
Commodity Contract Derivatives
Dec. 31, 2011
Commodity Contract Derivatives
Dec. 31, 2012
Interest Rate Swaption
Dec. 31, 2011
Interest Rate Swaption
Dec. 31, 2012
Minimum
tons-per-year
Dec. 31, 2012
Maximum
tons-per-year
Fair Value Measurements                    
Balances at beginning of period     $ 53,000,000 $ 22,000,000 $ (267,000,000) $ 239,000,000 $ 0 $ (1,077,000,000)    
Purchases     12,000,000 6,000,000 0 0 0 0    
Issuances     0 0 0 0 0 0    
Sales     (1,000,000) (1,000,000) 0 0 0 0    
Settlements     0 0 0 0 0 0    
Net realized (gains) losses recognized in income   0                
Net unrealized gains (losses) deferred as regulatory assets and liabilities     2,000,000 1,000,000 43,000,000 (235,000,000) 0 (51,000,000)    
Balances at end of period     66,000,000 28,000,000 (224,000,000) 4,000,000 0 (1,128,000,000)    
Proceeds from derivative instrument financing (60,000,000)                  
Assets                    
Fair value inputs, counterparty credit risk                 28.60% [1] 28.60% [1]
Fair value measurements tons per year                 1,000,000,000 1,100,000,000
Price per ton                 13.75 86.83
Liabilities                    
Fair value measurements tons per year                 1,000,000,000 1,100,000,000
Price per ton                 $ 13.75 $ 86.83
[1] Applies to only one contract.