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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, 2020
USD ($)
tons-per-year
Jun. 30, 2019
USD ($)
Jun. 30, 2020
USD ($)
tons-per-year
Jun. 30, 2019
USD ($)
Mar. 31, 2020
USD ($)
Sep. 30, 2019
USD ($)
tons-per-year
Mar. 31, 2019
USD ($)
Sep. 30, 2018
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Commodity contract derivatives $ 10,000,000   $ 10,000,000     $ 12,000,000    
Commodity contract derivatives 40,000,000   40,000,000     $ 53,000,000    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements $ (1,000,000)   $ (1,000,000)          
Maximum                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Fair value measurements tons per year | tons-per-year 0.6   0.6     0.8    
Price per ton $ 43.00   $ 43.00     $ 94.51    
Minimum                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Fair value measurements tons per year | tons-per-year 0.5   0.5     0.4    
Price per ton $ 11.90   $ 11.90     $ 12.10    
Fair Value, Inputs, Level 3                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Commodity contract derivatives 2,000,000   2,000,000     5,000,000    
Commodity contract derivatives 12,000,000   12,000,000     9,000,000    
Commodity Contract Derivatives                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Balance (10,000,000) $ 19,000,000 (10,000,000) $ 19,000,000 $ (7,000,000) $ (4,000,000) $ 38,000,000 $ 58,000,000
Net unrealized gains (losses) deferred as regulatory assets and liabilities $ (2,000,000) $ (19,000,000) $ (5,000,000) $ (39,000,000)