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Asset Retirement Obligations (Details) (USD $)
In Millions, unless otherwise specified
6 Months Ended
Mar. 31, 2013
Asset Retirement Obligations  
Asset retirement obligations, period increase (decrease) $ 66
Accretion expense 20
Balance at September 30, 2012 3,289 [1]
Settlements (ash storage areas) (15) [1]
Accretion (recorded as regulatory asset) 81 [1]
Balance at March 31, 2013 3,355 [1]
Current portion of ARO 31
Nuclear
 
Asset Retirement Obligations  
Balance at September 30, 2012 2,208 [1]
Settlements (ash storage areas) 0 [1]
Accretion (recorded as regulatory asset) 61 [1]
Balance at March 31, 2013 2,269 [1]
Non-nuclear
 
Asset Retirement Obligations  
Balance at September 30, 2012 1,081 [1]
Settlements (ash storage areas) (15) [1]
Accretion (recorded as regulatory asset) 20 [1]
Balance at March 31, 2013 $ 1,086 [1]
[1] The current portion of ARO in the amount of $31 million is included in Accounts payable and accrued liabilities.