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Asset Retirement Obligations (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Dec. 31, 2012
Asset Retirement Obligations  
Asset retirement obligations, period increase (decrease) $ 37
Accretion expense 10
Balance at September 30, 2012 3,289 [1]
Settlements (ash storage areas) (7) [1]
Accretion (recorded as regulatory asset) 44 [1]
Balance at December 31, 2012 3,326 [1]
Current portion of ARO 35
Nuclear
  
Asset Retirement Obligations  
Balance at September 30, 2012 2,208 [1]
Settlements (ash storage areas) 0 [1]
Accretion (recorded as regulatory asset) 30 [1]
Balance at December 31, 2012 2,238 [1]
Non-nuclear
  
Asset Retirement Obligations  
Balance at September 30, 2012 1,081 [1]
Settlements (ash storage areas) (7) [1]
Accretion (recorded as regulatory asset) 14 [1]
Balance at December 31, 2012 $ 1,088 [1]
[1] The current portion of ARO in the amount of $35 million is included in Accounts payable and accrued liabilities.