Asset Retirement Obligations Asset Retirement Obligations (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Liabilities Settled [Line Items]
Increase in ARO liability		$ (392)
Change in estimate	[1]	(561)	$ 132
Additional obligations			3,136
Balance	[1]	10,414	10,806	$ 7,489
Asset Retirement Obligation, Liabilities Settled	[1]	270	257
Accretion (recorded as regulatory asset)	[1]	439	306
Asset Retirement Obligation, Current		313	283
Amortization and Depreciation of Decontaminating and Decommissioning Assets		220	188	188
Change in estimate due to closure of coal yards		(34)	76
Change in estimate due to timing of asset retirement activities		27
Change in estimate due to Gallatin CCR closure			231
estimate revision of SLR		164
Increase in CCR post-closure care			32
Decrease for legacy CCR due to revised acreage assumptions			38
Change due to study of non-nuclear plant decommissioning obligations		27
Nuclear
Liabilities Settled [Line Items]
Change in estimate		2	(160)
Additional obligations			0
Balance		3,976	3,814	3,808
Asset Retirement Obligation, Liabilities Settled		15	4
Accretion (recorded as regulatory asset)		175	170
Non-nuclear
Liabilities Settled [Line Items]
Change in estimate		(563)	292
Additional obligations			3,136
Balance		6,438	6,992	$ 3,681
Asset Retirement Obligation, Liabilities Settled		255	253
Accretion (recorded as regulatory asset)		$ 264	$ 136

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