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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, 2018
USD ($)
tons-per-year
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
tons-per-year
Jun. 30, 2017
USD ($)
Mar. 31, 2018
USD ($)
Sep. 30, 2017
USD ($)
tons-per-year
Mar. 31, 2017
USD ($)
Sep. 30, 2016
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Commodity contract derivatives $ 27,000,000   $ 27,000,000     $ 10,000,000    
Commodity contract derivatives $ 31,000,000   $ 31,000,000     $ 70,000,000    
Maximum                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Fair value measurements tons per year | tons-per-year 0.8   0.8     0.7    
Price per ton $ 111.61   $ 111.61     $ 112.23    
Minimum                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Fair value measurements tons per year | tons-per-year 0.7   0.7     0.6    
Price per ton $ 12.35   $ 12.35     $ 11.40    
Fair Value, Inputs, Level 3                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Commodity contract derivatives 22,000,000   22,000,000     2,000,000    
Commodity contract derivatives 17,000,000   17,000,000     69,000,000    
Commodity Contract Derivatives                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Balance 5,000,000 $ (100,000,000) 5,000,000 $ (100,000,000) $ (52,000,000) $ (67,000,000) $ (112,000,000) $ (127,000,000)
Net unrealized gains (losses) deferred as regulatory assets and liabilities $ 57,000,000 $ 12,000,000 $ 72,000,000 $ 27,000,000