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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, 2016
USD ($)
tons-per-year
Dec. 31, 2015
USD ($)
Sep. 30, 2016
USD ($)
tons-per-year
Sep. 30, 2015
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Commodity contract derivatives $ 28,000,000   $ 9,000,000  
Commodity contract derivatives $ 115,000,000   $ 134,000,000  
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 $ 85.02   $ 85.02  
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 $ 11.65   $ 11.80  
Fair Value, Inputs, Level 3        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Commodity contract derivatives 9,000,000   4,000,000  
Commodity contract derivatives 113,000,000   131,000,000  
Commodity Contract Derivatives        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Balance (104,000,000) $ (123,000,000) $ (127,000,000) $ (98,000,000)
Net unrealized gains (losses) deferred as regulatory assets and liabilities $ 23,000,000 $ (25,000,000)