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Fair Value Measurements Fair Value Measurements - Fair Value Measurements Using Significant Unobservable Inputs (Details) (USD $)
 12 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Commodity contract derivatives $ 1,000,000 $ 3,000,000
Commodity contract derivatives 97,000,000 144,000,000
Liabilities    
Number of contracts 2 1
Fair Value Measurements    
Amount of gain (loss) recognized in income 57,000,000  
Private Partnerships
    
Fair Value Measurements    
Balances at beginning of period 159,000,000 53,000,000
Purchases 39,000,000 101,000,000
Issuances 0 0
Sales (9,000,000) (4,000,000)
Settlements 0 0
Net unrealized gains (losses) deferred as regulatory assets and liabilities 25,000,000 9,000,000
Balances at end of period 214,000,000 159,000,000
Commodity Contract Derivatives
    
Fair Value Measurements    
Balances at beginning of period (140,000,000) (267,000,000)
Purchases 0 0
Issuances 0 0
Sales 0 0
Settlements 33,000,000 0
Net unrealized gains (losses) deferred as regulatory assets and liabilities 11,000,000 127,000,000
Balances at end of period (96,000,000) (140,000,000)
Minimum
    
Assets    
Fair value inputs, counterparty credit risk 2.00%  
Fair value measurements tons per year 1,000,000,000 900,000,000
Price per ton 11.24 10.25
Liabilities    
Fair value measurements tons per year 1,000,000,000 900,000,000
Price per ton 11.24 10.25
Maximum
    
Assets    
Fair value inputs, counterparty credit risk 5.00% 21.00%
Fair value measurements tons per year 1,100,000,000 1,000,000,000
Price per ton 67.07 85.25
Liabilities    
Fair value measurements tons per year 1,100,000,000 1,000,000,000
Price per ton 67.07 85.25
Fair Value, Inputs, Level 3
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Commodity contract derivatives 1,000,000 3,000,000
Commodity contract derivatives $ 97,000,000 $ 143,000,000