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Fair Value Measurements Fair Value Measurements - Fair Value Measurements Using Significant Unobservable Inputs (Details)
tons-per-year in Billions
12 Months Ended
Sep. 30, 2018
USD ($)
tons-per-year
Sep. 30, 2017
USD ($)
tons-per-year
Sep. 30, 2016
USD ($)
Fair Value Measurements      
Commodity contract derivatives, assets $ 72,000,000 $ 10,000,000  
Commodity contract derivatives, liabilities $ 12,000,000 $ 70,000,000  
Minimum      
Assets      
Fair value measurements tons per year | tons-per-year 0.7 0.6  
Price per ton $ 12.25 $ 11.40  
Liabilities      
Fair value measurements tons per year | tons-per-year 0.7 0.6  
Price per ton $ 12.25 $ 11.40  
Maximum      
Assets      
Fair value measurements tons per year | tons-per-year 0.8 0.7  
Price per ton $ 112.24 $ 112.23  
Liabilities      
Fair value measurements tons per year | tons-per-year 0.8 0.7  
Price per ton $ 112.24 $ 112.23  
Commodity Contract Derivatives      
Fair Value Measurements      
Balance at beginning/end of period 58,000,000 (67,000,000) $ (127,000,000)
Purchases 0 0  
Issuances 0 0  
Sales 0 0  
Settlements 0 0  
Change in net unrealized gains (losses) deferred as regulatory assets and liabilities 125,000,000 60,000,000  
Fair Value, Inputs, Level 3      
Fair Value Measurements      
Commodity contract derivatives, assets 59,000,000 2,000,000  
Commodity contract derivatives, liabilities $ 1,000,000 $ 69,000,000