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Fair Value Measurements Fair Value Measurements Using Significant Unobservable Inputs (Details)
tons-per-year in Billions
3 Months Ended
Dec. 31, 2015
USD ($)
tons-per-year
Dec. 31, 2014
USD ($)
Sep. 30, 2015
USD ($)
tons-per-year
Sep. 30, 2014
USD ($)
ASSETS        
Commodity contract derivatives $ 4,000,000   $ 1,000,000  
Liabilities        
Commodity contract derivatives $ 142,000,000   $ 98,000,000  
Maximum        
ASSETS        
Fair value measurements tons per year | tons-per-year 1.0   1.0  
Price per ton $ 121.51   $ 103.41  
Liabilities        
Fair value measurements tons per year | tons-per-year 1.0   1.0  
Price per ton $ 121.51   $ 103.41  
Minimum        
ASSETS        
Fair value measurements tons per year | tons-per-year 0.8   0.8  
Price per ton $ 9.83   $ 10.64  
Liabilities        
Fair value measurements tons per year | tons-per-year 0.8   0.8  
Price per ton $ 9.83   $ 10.64  
Fair Value, Inputs, Level 3        
ASSETS        
Commodity contract derivatives 0   0  
Liabilities        
Commodity contract derivatives 123,000,000   98,000,000  
Commodity Contract Derivatives        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Balance (123,000,000) $ (107,000,000) $ (98,000,000) $ (85,000,000)
Purchases 0 0    
Issuances 0 0    
Sales 0 0    
Settlements 0 0    
Net unrealized gains (losses) deferred as regulatory assets and liabilities $ (25,000,000) $ (22,000,000)