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Summary of Significant Accounting Policies - Property, Plant, and Equipment, and Depreciation (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Property, Plant, and Equipment, and Depreciation      
Depreciation $ 1,400 $ 1,600 $ 1,800
Composite depreciation rate for completed plant 2.28% 2.74% 3.09%
Reacquired Rights $ 184 $ 192  
Amortization of Reacquired Rights $ 8 8 $ 8
Capitalized software amortization period 7 years    
Unamortized computer software costs $ 27 54  
Amortization expense of capitalized computer software costs 38 42 38
Accelerated depreciation 136 $ 387 $ 566
Increase (decrease) in Year 1 due to new depreciation study $ 369    
Nuclear      
Property, Plant, and Equipment, and Depreciation      
Composite depreciation rate for completed plant 2.38% 2.38% 2.38%
Coal-fired      
Property, Plant, and Equipment, and Depreciation      
Composite depreciation rate for completed plant 1.95% 3.62% 4.96%
Hydroelectric      
Property, Plant, and Equipment, and Depreciation      
Composite depreciation rate for completed plant 1.60% 1.60% 1.61%
Gas and oil-fired      
Property, Plant, and Equipment, and Depreciation      
Composite depreciation rate for completed plant 2.98% 3.04% 3.00%
Transmission      
Property, Plant, and Equipment, and Depreciation      
Composite depreciation rate for completed plant 1.34% 1.34% 1.34%
Other      
Property, Plant, and Equipment, and Depreciation      
Composite depreciation rate for completed plant 7.12% 7.26% 7.16%