Summary of Significant Accounting Policies - Property, Plant, and Equipment, and Depreciation (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Property, Plant, and Equipment, and Depreciation
Depreciation	$ 1,800	$ 1,400	$ 1,600
Composite depreciation rate for completed plant	2.98%	2.28%	2.74%
Reacquired Rights	$ 178	$ 184
Amortization of Reacquired Rights	$ 6	8	$ 8
Capitalized software amortization period	7 years
Accelerated depreciation	$ 140	136	$ 125
Increase (decrease) in Year 1 due to new depreciation study	345
Accumulated Amortization of Reacquired Rights	42	$ 36
Amortization Reacquired Rights, Year 1	6
Amortization Reacquired Rights, Year 2	6
Amortization Reacquired Rights, Year 3	6
Amortization Reacquired Rights, Year 4	6
Amortization Reacquired Rights, Year 5	6
Amortization Reacquired Rights, Year Thereafter	$ 148
Nuclear
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	2.72%	2.38%	2.38%
Coal-fired
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	4.27%	1.95%	3.62%
Hydroelectric
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	1.85%	1.60%	1.60%
Gas and oil-fired
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	3.38%	2.98%	3.04%
Transmission
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	1.51%	1.34%	1.34%
Other
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	3.64%	7.12%	7.26%

Property, Plant, and Equipment, and Depreciation