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Fair Value Measurements Fair Value Measurements Using Significant Unobservable Inputs (Details)
tons-per-year in Billions
3 Months Ended 9 Months Ended
Jun. 30, 2016
USD ($)
tons-per-year
Jun. 30, 2015
USD ($)
Jun. 30, 2016
USD ($)
tons-per-year
Jun. 30, 2015
USD ($)
Mar. 31, 2016
USD ($)
Sep. 30, 2015
USD ($)
tons-per-year
Mar. 31, 2015
USD ($)
Sep. 30, 2014
USD ($)
ASSETS                
Commodity contract derivatives $ 12,000,000   $ 12,000,000     $ 1,000,000    
Liabilities                
Commodity contract derivatives $ 189,000,000   $ 189,000,000     $ 98,000,000    
Maximum                
ASSETS                
Fair value measurements tons per year | tons-per-year 0.7   0.7     1.0    
Price per ton $ 76.25   $ 76.25     $ 103.41    
Liabilities                
Fair value measurements tons per year | tons-per-year 0.7   0.7     1.0    
Price per ton $ 76.25   $ 76.25     $ 103.41    
Minimum                
ASSETS                
Fair value measurements tons per year | tons-per-year 0.4   0.4     0.8    
Price per ton $ 9.45   $ 9.45     $ 10.64    
Liabilities                
Fair value measurements tons per year | tons-per-year 0.4   0.4     0.8    
Price per ton $ 9.45   $ 9.45     $ 10.64    
Fair Value, Inputs, Level 3                
ASSETS                
Commodity contract derivatives 0   0     0    
Liabilities                
Commodity contract derivatives 183,000,000   183,000,000     98,000,000    
Commodity Contract Derivatives                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Balance (183,000,000) $ (128,000,000) (183,000,000) $ (128,000,000) $ (187,000,000) $ (98,000,000) $ (154,000,000) $ (85,000,000)
Purchases 0 0 0 0        
Issuances 0 0 0 0        
Sales 0 0 0 0        
Settlements 0 0 0 0        
Net unrealized gains (losses) deferred as regulatory assets and liabilities $ 4,000,000 $ 26,000,000 $ (85,000,000) $ (43,000,000)