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Fair Value Measurements Fair Value Measurements Using Significant Unobservable Inputs (Details)
tons-per-year in Billions
 3 Months Ended 6 Months Ended
Mar. 31, 2019
USD ($)
tons-per-year
Mar. 31, 2018
USD ($)
Mar. 31, 2019
USD ($)
tons-per-year
Mar. 31, 2018
USD ($)
Dec. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
tons-per-year
Dec. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Commodity contract derivatives $ 42,000,000   $ 42,000,000     $ 72,000,000    
Commodity contract derivatives $ 14,000,000   $ 14,000,000     $ 12,000,000    
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     0.8    
Price per ton $ 112.64   $ 112.64     $ 112.24    
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     0.7    
Price per ton $ 12.25   $ 12.25     $ 12.25    
Fair Value, Inputs, Level 3                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Commodity contract derivatives 40,000,000   40,000,000     59,000,000    
Commodity contract derivatives (2,000,000)   (2,000,000)     1,000,000    
Commodity Contract Derivatives                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Balance 38,000,000 $ (52,000,000) 38,000,000 $ (52,000,000) $ 63,000,000 $ 58,000,000 $ (73,000,000) $ (67,000,000)
Net unrealized gains (losses) deferred as regulatory assets and liabilities $ (25,000,000) $ 21,000,000 $ (20,000,000) $ 15,000,000