Commitments and Contingencies - Table (Details) $ in Millions		12 Months Ended
		Sep. 30, 2025 USD ($) megawatts	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)
Obligations
Megawatts provided under transmission obligations | megawatts		3,750
Accrual for Environmental Loss Contingencies, Gross		$ 8	$ 15
Estimated future decommissioning cost	[1]	$ 10,414	10,806	$ 7,489
Remaining terms of the transmission obligations, high end of range		6 years
Nuclear
Obligations
Estimated future decommissioning cost		$ 3,976	$ 3,814	$ 3,808

[1] (1) Includes $313 million and $283 million at September 30, 2025 and 2024, respectively, in Current liabilities.