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Asset Retirement Obligations (Details) (USD $)
In Millions, unless otherwise specified
 9 Months Ended
Jun. 30, 2013
Asset Retirement Obligations  
Asset retirement obligations, period increase (decrease) $ 149
Accretion expense 30
Balance at September 30, 2012 3,289 [1]
Settlements (ash storage areas) (25) [1]
Accretion (recorded as regulatory asset) 126 [1]
Change in estimate 48 [1]
Balance at June 30, 2013 3,438 [1]
Current portion of ARO 34
Nuclear
  
Asset Retirement Obligations  
Balance at September 30, 2012 2,208 [1]
Settlements (ash storage areas) 0 [1]
Accretion (recorded as regulatory asset) 93 [1]
Change in estimate 48 [1]
Balance at June 30, 2013 2,349 [1]
Non-nuclear
  
Asset Retirement Obligations  
Balance at September 30, 2012 1,081 [1]
Settlements (ash storage areas) (25) [1]
Accretion (recorded as regulatory asset) 33 [1]
Change in estimate 0 [1]
Balance at June 30, 2013 $ 1,089 [1]
[1] The current portion of ARO in the amount of $34 million is included in Accounts payable and accrued liabilities at June 30, 2013.