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Fair Value Measurements Fair Value Measurements Using Significant Unobservable Inputs (Details) (USD $)
 3 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Sep. 30, 2013
Sep. 30, 2012
ASSETS        
Commodity contract derivatives $ 4,000,000   $ 3,000,000  
Liabilities        
Commodity contract derivatives 150,000,000   144,000,000  
Number of contracts 1      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Amount of gain (loss) recognized in income 0      
Partnership Interest
        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Balance 169,000,000 66,000,000 159,000,000 53,000,000
Purchases 8,000,000 12,000,000    
Issuances 0 0    
Sales (2,000,000) (1,000,000)    
Settlements 0 0    
Net unrealized gains (losses) deferred as regulatory assets and liabilities 4,000,000 2,000,000    
Commodity Contract Derivatives
        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Balance (145,000,000) (224,000,000) (140,000,000) (267,000,000)
Purchases 0 0    
Issuances 0 0    
Sales 0 0    
Settlements 0 0    
Net unrealized gains (losses) deferred as regulatory assets and liabilities 5,000,000 43,000,000    
Maximum
        
ASSETS        
Fair value inputs, counterparty credit risk 18.00% [1]      
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      
Minimum
        
ASSETS        
Fair value inputs, counterparty credit risk 18.00% [1]      
Fair value measurements tons per year 900,000,000      
Price per ton 11.90      
Liabilities        
Fair value measurements tons per year 900,000,000      
Price per ton 11.90      
Fair Value, Inputs, Level 3
        
ASSETS        
Commodity contract derivatives 4,000,000   3,000,000  
Liabilities        
Commodity contract derivatives $ 149,000,000   $ 143,000,000  
[1] Applies to only one contract.