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Summary of Significant Accounting Policies - Property, Plant, and Equipment, and Depreciation (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2021
Property, Plant, and Equipment, and Depreciation      
Depreciation $ 1,900 $ 1,800 $ 1,400
Composite depreciation rate for completed plant 3.14% 2.98% 2.28%
Reacquired Rights $ 324 $ 178  
Amortization of Reacquired Rights $ 10 6 $ 8
Capitalized software amortization period 7 years    
Accelerated depreciation $ 177 140 $ 136
Accumulated Amortization of Reacquired Rights 52 $ 42  
Amortization Reacquired Rights, Year 1 11    
Amortization Reacquired Rights, Year 2 12    
Amortization Reacquired Rights, Year 3 11    
Amortization Reacquired Rights, Year 4 12    
Amortization Reacquired Rights, Year 5 11    
Amortization Reacquired Rights, Year Thereafter 267    
Reimbursements from DOE 219    
Accounts receivable from DOE $ 11    
Nuclear      
Property, Plant, and Equipment, and Depreciation      
Composite depreciation rate for completed plant 2.73% 2.72% 2.38%
Coal-fired      
Property, Plant, and Equipment, and Depreciation      
Composite depreciation rate for completed plant 4.98% 4.27% 1.95%
Hydroelectric      
Property, Plant, and Equipment, and Depreciation      
Composite depreciation rate for completed plant 1.82% 1.85% 1.60%
Gas and oil-fired      
Property, Plant, and Equipment, and Depreciation      
Composite depreciation rate for completed plant 3.17% 3.38% 2.98%
Transmission      
Property, Plant, and Equipment, and Depreciation      
Composite depreciation rate for completed plant 1.52% 1.51% 1.34%
Other      
Property, Plant, and Equipment, and Depreciation      
Composite depreciation rate for completed plant 4.43% 3.64% 7.12%