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Risk Management Activities and Derivative Transactions Derivatives Under FTP (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2013
Sep. 30, 2013
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  
Swap contracts
    
Derivative    
Remaining terms 4 years  
Coal Contract Derivatives
    
Derivative    
Fair value $ 0 $ 0