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Risk Management Activities and Derivative Transactions Derivatives Under FTP (Details) (USD $)
In Millions, unless otherwise specified
 9 Months Ended
Jun. 30, 2014
Jun. 30, 2014
Swap contracts
Jun. 30, 2014
Coal Contract Derivatives
Sep. 30, 2013
Coal Contract Derivatives
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      
Fair value     $ 0 $ 0
Remaining terms   4 years