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Summary of Significant Accounting Policies - Property, Plant, and Equipment, and Depreciation (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Property, Plant, and Equipment, and Depreciation      
Depreciation $ 1,600 $ 1,800 $ 1,300
Composite depreciation rate for completed plant 2.74% 3.09% 2.45%
Accelerated depreciation   $ 566 $ 48
Reacquired Rights $ 192 200  
Amortization of Reacquired Rights $ 8 8 8
Capitalized software amortization period seven years    
Unamortized computer software costs $ 54 63  
Amortization expense of capitalized computer software costs $ 42 $ 38 $ 32
Nuclear      
Property, Plant, and Equipment, and Depreciation      
Composite depreciation rate for completed plant 2.38% 2.38% 2.64%
Coal-fired      
Property, Plant, and Equipment, and Depreciation      
Composite depreciation rate for completed plant 3.62% 4.96% 2.32%
Hydroelectric      
Property, Plant, and Equipment, and Depreciation      
Composite depreciation rate for completed plant 1.60% 1.61% 1.57%
Gas and oil-fired      
Property, Plant, and Equipment, and Depreciation      
Composite depreciation rate for completed plant 3.04% 3.00% 2.93%
Transmission      
Property, Plant, and Equipment, and Depreciation      
Composite depreciation rate for completed plant 1.34% 1.34% 1.32%
Other      
Property, Plant, and Equipment, and Depreciation      
Composite depreciation rate for completed plant 7.26% 7.16% 5.90%