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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, 2017
USD ($)
tons-per-year
Dec. 31, 2016
USD ($)
Sep. 30, 2017
USD ($)
tons-per-year
Sep. 30, 2016
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Commodity contract derivatives $ 16,000,000   $ 10,000,000  
Commodity contract derivatives $ 93,000,000   $ 70,000,000  
Maximum        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fair value measurements tons per year | tons-per-year 0.7   0.7  
Price per ton $ 112.81   $ 112.23  
Minimum        
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  
Price per ton $ 12.15   $ 11.40  
Fair Value, Inputs, Level 3        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Commodity contract derivatives 9,000,000   2,000,000  
Commodity contract derivatives 82,000,000   69,000,000  
Commodity Contract Derivatives        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Balance (73,000,000) $ (104,000,000) $ (67,000,000) $ (127,000,000)
Net unrealized gains (losses) deferred as regulatory assets and liabilities $ (6,000,000) $ 23,000,000