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Fair Value Measurements Fair Value Measurements Using Significant Unobservable Inputs (Details) (USD $)
 3 Months Ended 9 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Fair Value Measurements        
Net realized (gains) losses recognized in income $ 0   $ 0  
Unrealized gain (losses) recognized in OCI 0 0 0 0
Private Partnerships
        
Fair Value Measurements        
Balances at beginning of period 36,000,000 14,000,000 22,000,000 13,000,000
Purchases 9,000,000 4,000,000 21,000,000 13,000,000
Issuances 0 0 0 0
Sales 0 0 (2,000,000) 0
Settlements 0 0 0 (7,000,000)
Net realized (gains) losses recognized in income 0   0  
Unrealized gain (losses) recognized in OCI 0   0  
Net unrealized gains (losses) deferred as regulatory assets and liabilities 0 0 4,000,000 (1,000,000)
Balances at end of period 45,000,000 18,000,000 45,000,000 18,000,000
Commodity Contract Derivatives
        
Fair Value Measurements        
Balances at beginning of period (311,000,000) 73,000,000 239,000,000 103,000,000
Purchases 0 0 0 0
Issuances 0 0 0 0
Sales 0 0 0 0
Settlements 0 0 0 0
Net realized (gains) losses recognized in income 0   0  
Unrealized gain (losses) recognized in OCI 0   0  
Net unrealized gains (losses) deferred as regulatory assets and liabilities (20,000,000) 51,000,000 (570,000,000) 21,000,000
Balances at end of period (331,000,000) 124,000,000 (331,000,000) 124,000,000
Swaption
        
Fair Value Measurements        
Balances at beginning of period (993,000,000) (554,000,000) (1,077,000,000) (804,000,000)
Purchases 0 0 0 0
Issuances 0 0 0 0
Sales 0 0 0 0
Settlements 993,000,000 0 1,077,000,000 0
Net realized (gains) losses recognized in income 0   0  
Unrealized gain (losses) recognized in OCI 0   0  
Net unrealized gains (losses) deferred as regulatory assets and liabilities 0 (75,000,000) 0 175,000,000
Balances at end of period 0 (629,000,000) 0 (629,000,000)
Minimum
        
Assets        
Fair value inputs, counterparty credit risk     28.60%  
Fair value measurements tons per year 1,000,000,000   1,000,000,000  
Price per ton 13.50   13.50  
Liabilities        
Fair value measurements tons per year 1,000,000,000   1,000,000,000  
Price per ton 13.50   13.50  
Maximum
        
Assets        
Fair value inputs, counterparty credit risk     28.60%  
Fair value measurements tons per year 1,100,000,000   1,100,000,000  
Price per ton 93.00   93.00  
Liabilities        
Fair value measurements tons per year 1,100,000,000   1,100,000,000  
Price per ton $ 93.00   $ 93.00