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Fair Value Measurements Fair Value Measurements Using Significant Unobservable Inputs (Details)
tons-per-year in Billions
 3 Months Ended
Dec. 31, 2019
USD ($)
tons-per-year
Dec. 31, 2018
USD ($)
Sep. 30, 2019
USD ($)
tons-per-year
Mar. 31, 2018
USD ($)
Sep. 30, 2017
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Commodity contract derivatives $ 4,000,000   $ 12,000,000    
Commodity contract derivatives $ 66,000,000   $ 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    
Price per ton $ 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    
Price per ton $ 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   5,000,000    
Commodity contract derivatives 18,000,000   9,000,000    
Commodity Contract Derivatives          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Balance (17,000,000)   $ (4,000,000) $ 63,000,000 $ 58,000,000
Net unrealized gains (losses) deferred as regulatory assets and liabilities $ (13,000,000) $ 5,000,000