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Regulatory Assets and Liabilities Regulatory Assets and Liabilities (Details) (USD $)
 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Jul. 31, 2005
Units
Sep. 30, 2013
Deferred Nuclear Generating Units and Construction Costs
Jul. 31, 2005
Deferred Nuclear Generating Units and Construction Costs
Sep. 30, 2013
Environmental Cleanup Costs - Kingston Ash Spill
Sep. 30, 2010
Environmental Cleanup Costs - Kingston Ash Spill
Sep. 30, 2009
Environmental Cleanup Costs - Kingston Ash Spill
Sep. 30, 2013
Environmental Agreements
Sep. 30, 2013
Energy Efficiency Projects
Environmental Agreements
Sep. 30, 2013
Paid to States
Environmental Agreements
Sep. 30, 2013
Civil Penalties
Environmental Agreements
Sep. 30, 2013
Derivatives Under FTP
Sep. 30, 2012
Derivatives Under FTP
Sep. 30, 2013
Deferred nuclear generating units
Regulatory Assets and Liabilities                              
Net unrealized gains (losses)                         $ 166,000,000 [1] $ 228,000,000 [1]  
Investment in deferred generating units         3,900,000,000                    
Number of deferred generating units     2                        
Regulatory Noncurrent Asset, Amortization Period       P10Y   P15Y                  
Remaining balance to be included in plant asset balance at completion       619,000,000                      
Regulatory Assets 9,692,000,000 11,901,000,000                         1,700,000,000
Yearly amortization of Deferred nuclear generating units 237,000,000                            
Initial regulatory asset                 360,000,000            
Investment in energy efficiency projects                 290,000,000            
Amount to be provided to fund environmental projects                 60,000,000            
Amount to be paid in civil penalties                 10,000,000            
Regulatory asset amount expensed                   52,000,000 36,000,000 10,000,000      
Total recorded estimate of Kingston cleanup costs 1,100,000,000         1,100,000,000   933,000,000              
Estimate increase             $ 192,000,000                
Current regulatory asset amortization period           12 months                  
[1] The September 30, 2013, and September 30, 2012 balances in the Derivatives Under Financial Trading Program table show all open derivative positions in the FTP.