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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Derivative Instruments That Do Not Receive Hedge Accounting Treatment (Details)
 12 Months Ended
Sep. 30, 2016
USD ($)
Sep. 30, 2015
USD ($)
Derivative    
Unrealized gains/losses on derivatives $ 0 $ 0
Change in Unrealized gains (losses) on Interest Rate Derivatives 311,000,000 279,000,000
FTP transaction limit $ 130,000,000  
Maximum hedge volume 75.00%  
Market value limitation of outstanding construction materials hedging transactions $ 100,000,000  
Portfolio value at risk limit for foreign currency transactions 5,000,000  
Interest Rate Swap    
Derivative    
Amount of gain (loss) recognized in income on derivatives (109,000,000) (114,000,000)
Commodity Contract Derivatives    
Derivative    
Fair value (125,000,000) (97,000,000)
Commodity derivatives under FTP    
Derivative    
Amount of gain (loss) recognized in income on derivatives $ (94,000,000) $ (98,000,000)
Coal Contract Derivatives    
Derivative    
Number of contracts 20 14
Notional amount 20,000,000 19,000,000
Fair value $ (127,000,000) $ (98,000,000)
Natural Gas Contract Derivatives    
Derivative    
Number of contracts 39 33
Notional amount 148,000,000 134,000,000
Fair value $ 2,000,000 $ 1,000,000
Swap contracts    
Derivative    
Remaining terms 2  
Maximum | Coal Contract Derivatives    
Derivative    
Derivative, term 3 years  
Maximum | Natural Gas Contract Derivatives    
Derivative    
Derivative, term 3 years