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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, 2019
USD ($)
tons-per-year
Jun. 30, 2018
USD ($)
Jun. 30, 2019
USD ($)
tons-per-year
Jun. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
tons-per-year
Mar. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                  
Commodity contract derivatives $ 20,000,000   $ 20,000,000     $ 72,000,000      
Commodity contract derivatives $ 42,000,000   $ 42,000,000     $ 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 19,000,000   19,000,000     59,000,000      
Commodity contract derivatives 0   0     1,000,000      
Commodity Contract Derivatives                  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                  
Balance 19,000,000   19,000,000   $ 38,000,000 $ 58,000,000 $ 5,000,000 $ (52,000,000) $ (67,000,000)
Net unrealized gains (losses) deferred as regulatory assets and liabilities $ (19,000,000) $ 57,000,000 $ (39,000,000) $ 72,000,000