Summary of Significant Accounting Policies - Property, Plant, and Equipment, and Depreciation (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Property, Plant, and Equipment, and Depreciation
Depreciation	$ 1,900	$ 1,800	$ 1,900
Composite depreciation rate for completed plant	3.04%	2.92%	3.14%
Reacquired Rights	$ 301	$ 312
Amortization of Reacquired Rights	$ 11	11	$ 10
Capitalized software amortization period	7 years
Accumulated Amortization of Reacquired Rights	$ 74	$ 63
Amortization Reacquired Rights, Year 1	11
Amortization Reacquired Rights, Year 2	12
Amortization Reacquired Rights, Year 3	11
Amortization Reacquired Rights, Year 4	11
Amortization Reacquired Rights, Year 5	12
Amortization Reacquired Rights, Year Thereafter	244
Reimbursements from DOE	334
Accounts receivable from DOE	$ 6
Nuclear
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	2.71%	2.71%	2.73%
Coal-fired
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	4.82%	4.13%	4.98%
Hydroelectric
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	1.82%	1.83%	1.82%
Gas and oil-fired
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	3.16%	3.31%	3.17%
Transmission
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	1.55%	1.52%	1.52%
Other
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	5.22%	4.98%	4.43%