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Fair Value Measurements Fair Value Measurements - Fair Value Measurements Using Significant Unobservable Inputs (Details) (USD $)
 12 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Commodity contact derivatives $ 3,000,000 $ 119,000,000
Commodity contract derivatives 144,000,000 386,000,000
Liabilities    
Number of contracts 1  
Fair Value Measurements    
Amount of gain (loss) recognized in income 0  
Private Partnerships
    
Fair Value Measurements    
Balances at beginning of period 53,000,000 22,000,000
Purchases 101,000,000 27,000,000
Issuances 0 0
Sales (4,000,000) 0
Settlements 0 0
Net unrealized gains (losses) deferred as regulatory assets and liabilities 9,000,000 4,000,000
Balances at end of period 159,000,000 53,000,000
Commodity Contract Derivatives
    
Fair Value Measurements    
Balances at beginning of period (267,000,000) 239,000,000
Purchases 0 0
Issuances 0 0
Sales 0 0
Settlements 0 0
Net unrealized gains (losses) deferred as regulatory assets and liabilities 127,000,000 (506,000,000)
Balances at end of period (140,000,000) (267,000,000)
Interest Rate Swaption
    
Fair Value Measurements    
Balances at beginning of period 0 (1,077,000,000)
Purchases 0 0
Issuances 0 0
Sales 0 0
Settlements 0 [1] 1,077,000,000
Net unrealized gains (losses) deferred as regulatory assets and liabilities 0 0
Balances at end of period 0 0
Minimum
    
Assets    
Fair value measurements tons per year 900,000,000  
Price per ton 10.25  
Liabilities    
Fair value measurements tons per year 900,000,000  
Price per ton 10.25  
Maximum
    
Assets    
Fair value inputs, counterparty credit risk 21.00% [2]  
Fair value measurements tons per year 1,000,000,000  
Price per ton 85.25  
Liabilities    
Fair value measurements tons per year 1,000,000,000  
Price per ton 85.25  
Fair Value, Inputs, Level 3
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Commodity contact derivatives 3,000,000 119,000,000
Commodity contract derivatives $ 143,000,000 $ 386,000,000
[1] The interest rate swaption was converted to an interest rate swap in April 2012.
[2] Applies to only one contract.