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Risk Management Activities and Derivative Transactions Derivatives Under FTP (Details)
$ in Millions
 3 Months Ended
Dec. 31, 2015
USD ($)
Sep. 30, 2015
USD ($)
Derivative    
FTP transaction limit $ 130  
Maximum hedge volume 75.00%  
Market value limitation of outstanding construction materials hedging transactions $ 100  
Portfolio value at risk limit for foreign currency transactions $ 5  
Natural Gas    
Derivative    
Notional amount 132,000,000 134,000,000
Fair value $ (15) $ 1
Number of Price Risk Derivatives Held 30 33
Coal Contract    
Derivative    
Notional amount 20,000,000 19,000,000
Fair value $ (123) $ (98)
Number of Price Risk Derivatives Held 12 14
Swap contracts    
Derivative    
Remaining terms 2  
Natural Gas Swap    
Derivative    
Notional amount [1] 40,977,500 51,495,000
Fair value [1] $ (101) $ (116)
Maximum | Natural Gas    
Derivative    
Derivative, Term of Contract 3 years  
Maximum | Coal Contract    
Derivative    
Derivative, Term of Contract 3 years  
[1] Fair value amounts presented are based on the net commodity position with the counterparty. Notional amounts disclosed represent the net absolute value of contractual amounts.