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Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Property, Plant, and Equipment, and Depreciation (Details)
$ in Millions
 12 Months Ended
Sep. 30, 2016
USD ($)
Sep. 30, 2015
USD ($)
Sep. 30, 2014
USD ($)
Jun. 30, 2011
Units
Property, Plant, and Equipment, and Depreciation        
Depreciation $ 1,400 $ 1,700 $ 1,600  
Composite depreciation rate for completed plant 2.97% 3.71% 3.42%  
Change in Depreciation Due to License Change $ 79      
Accelerated depreciation 139 $ 383 $ 206  
CGLP Capital lease liability 76      
Allowance for funds used during construction 235 214    
AFUDC minimum total project cost $ 1,000      
Minimum construction period 3 years      
Accumulated balance of qualifying projects $ 4,500      
Capitalized software amortization period 5 years      
Unamortized computer software costs $ 27 18    
Amortization expense of capitalized computer software costs 43 38 $ 31  
Electricity Generation Plant, Non-Nuclear        
Property, Plant, and Equipment, and Depreciation        
Capital Leases, Net $ 163 $ 94    
Nuclear        
Property, Plant, and Equipment, and Depreciation        
Composite depreciation rate for completed plant 2.37% 2.81% 2.90%  
Coal-fired        
Property, Plant, and Equipment, and Depreciation        
Composite depreciation rate for completed plant 3.50% 5.50% 4.37%  
Hydroelectric        
Property, Plant, and Equipment, and Depreciation        
Composite depreciation rate for completed plant 1.29% 1.30% 1.44%  
Gas and oil-fired        
Property, Plant, and Equipment, and Depreciation        
Composite depreciation rate for completed plant 3.09% 3.18% 3.23%  
Transmission        
Property, Plant, and Equipment, and Depreciation        
Composite depreciation rate for completed plant 2.80% 2.78% 2.76%  
Other        
Property, Plant, and Equipment, and Depreciation        
Composite depreciation rate for completed plant 8.97% 8.65% 8.40%  
Environmental Agreements        
Property, Plant, and Equipment, and Depreciation        
Number of units to be idled | Units       18