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Asset Retirement Obligations Asset Retirement Obligations (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Asset Retirement Obligations    
Increase in ARO liability $ 674  
Initial decommissioning estimate for nuclear 7 years  
ARO-Nuclear Decommissioning Study Update 5 years  
Accretion expense $ 44 $ 40
Balance [1] 3,169 3,488
Settlements (ash storage areas) [1] (58) (14)
Change in estimate (coal combustion residuals rule)) [1] 469  
Change in estimate (nuclear license extension) [1] 36  
Change in estimate [1] (25) (10)
Additional obligations [1] 101  
Accretion (recorded as regulatory asset) [1] 151 177
Change in estimate (nuclear site - specific studies) [1]   (472)
Balance [1] 3,843 3,169
Current portion of ARO 161 80
Nuclear    
Asset Retirement Obligations    
Balance [1] 2,052 2,399
Settlements (ash storage areas) [1] 0 0
Change in estimate (coal combustion residuals rule)) [1] 0  
Change in estimate (nuclear license extension) [1] 36  
Change in estimate [1] 0 0
Additional obligations [1] 0  
Accretion (recorded as regulatory asset) [1] 99 125
Change in estimate (nuclear site - specific studies) [1]   (472)
Balance [1] 2,187 2,052
Non-nuclear    
Asset Retirement Obligations    
Balance [1] 1,117 1,089
Settlements (ash storage areas) [1] (58) (14)
Change in estimate (coal combustion residuals rule)) [1] 469  
Change in estimate (nuclear license extension) [1] 0  
Change in estimate [1] (25) (10)
Additional obligations [1] 101  
Accretion (recorded as regulatory asset) [1] 52 52
Change in estimate (nuclear site - specific studies) [1]   0
Balance [1] 1,656 $ 1,117
Plant acquisition [Member] | Non-nuclear    
Asset Retirement Obligations    
Additional obligations 7  
Coal-fired obligations [Member] | Non-nuclear    
Asset Retirement Obligations    
Additional obligations $ 94  
[1] The current portions of ARO in the amounts of $161 million and $80 million as of September 30, 2015 and 2014, respectively, are included in Accounts payable and accrued liabilities.