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Summary of Significant Accounting Policies - Property, Plant, and Equipment, and Depreciation (Details) - USD ($)
$ in Millions
 12 Months Ended 57 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Property, Plant, and Equipment, and Depreciation        
Depreciation $ 1,800 $ 1,900 $ 1,800  
Composite depreciation rate for completed plant 2.92% 3.14% 2.98%  
Reacquired Rights $ 312 $ 324   $ 324
Amortization of Reacquired Rights $ 11 10 $ 6  
Capitalized software amortization period 7 years      
Accelerated depreciation   177 $ 140 659
Accumulated Amortization of Reacquired Rights $ 63 $ 52   $ 52
Amortization Reacquired Rights, Year 1 12      
Amortization Reacquired Rights, Year 2 11      
Amortization Reacquired Rights, Year 3 12      
Amortization Reacquired Rights, Year 4 11      
Amortization Reacquired Rights, Year 5 11      
Amortization Reacquired Rights, Year Thereafter 255      
Reimbursements from DOE 284      
Accounts receivable from DOE $ 13      
Nuclear        
Property, Plant, and Equipment, and Depreciation        
Composite depreciation rate for completed plant 2.71% 2.73% 2.72%  
Coal-fired        
Property, Plant, and Equipment, and Depreciation        
Composite depreciation rate for completed plant 4.13% 4.98% 4.27%  
Hydroelectric        
Property, Plant, and Equipment, and Depreciation        
Composite depreciation rate for completed plant 1.83% 1.82% 1.85%  
Gas and oil-fired        
Property, Plant, and Equipment, and Depreciation        
Composite depreciation rate for completed plant 3.31% 3.17% 3.38%  
Transmission        
Property, Plant, and Equipment, and Depreciation        
Composite depreciation rate for completed plant 1.52% 1.52% 1.51%  
Other        
Property, Plant, and Equipment, and Depreciation        
Composite depreciation rate for completed plant 4.98% 4.43% 3.64%