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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, 2020
USD ($)
tons-per-year
Mar. 31, 2019
USD ($)
Mar. 31, 2020
USD ($)
tons-per-year
Mar. 31, 2019
USD ($)
Dec. 31, 2019
USD ($)
Sep. 30, 2019
USD ($)
tons-per-year
Dec. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Commodity contract derivatives $ 4,000,000   $ 4,000,000     $ 12,000,000    
Commodity contract derivatives $ 60,000,000   $ 60,000,000     $ 53,000,000    
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 1,000,000   1,000,000     5,000,000    
Commodity contract derivatives 8,000,000   8,000,000     9,000,000    
Commodity Contract Derivatives                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Balance (7,000,000) $ 38,000,000 (7,000,000) $ 38,000,000 $ (17,000,000) $ (4,000,000) $ 63,000,000 $ 58,000,000
Net unrealized gains (losses) deferred as regulatory assets and liabilities $ 10,000,000 $ (25,000,000) $ (3,000,000) $ (20,000,000)