Revenue Customer Type (Details) $ in Millions	12 Months Ended
	Sep. 30, 2025 USD ($) Units	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)
Electric revenue	$ 13,483	$ 12,123	$ 11,888
Bill credits for LTA	$ (231)	(215)	199
Percentage of total operating revenues	90.20%
Total number of LPCs | Units	153
Revenue capitalized during pre-commercial operations	$ (4)	(3)	(3)
Pandemic Relief Credit	$ 225
Number of LPCs signed flexibility agreement | Units	109
Regulated Operating Revenue	$ 13,486	12,128	11,899
Regulated and Unregulated Operating Revenue	$ 13,672	$ 12,314	$ 12,054
Percent of sales of electricity to LPCs	91.00%	92.00%	92.00%
MLGW's % of operating revenues	9.00%	9.00%	9.00%
NES's % of operating revenues	8.00%	8.00%	8.00%
Other Operating Segment
Unregulated Operating Revenue	$ (186)	$ (186)	$ (155)
Off-System Sales
Unregulated Operating Revenue	(7)	(8)	(14)
Pre-commercial Operations - Capitalized Revenue
Unregulated Operating Revenue	4	(3)	(3)
ALABAMA
Regulated Operating Revenue	1,995	1,768	1,731
GEORGIA
Regulated Operating Revenue	329	295	284
KENTUCKY
Regulated Operating Revenue	850	776	773
MISSISSIPPI
Regulated Operating Revenue	1,254	1,150	1,146
NORTH CAROLINA
Regulated Operating Revenue	88	89	89
TENNESSEE
Regulated Operating Revenue	8,915	7,998	7,819
VIRGINIA
Regulated Operating Revenue	$ 52	47	46
5-year contract arrangement [Member]
Number of LPCs signed LTA | Units	5
Percentage of total operating revenues	12.40%
Revenues	$ 1,698
20-year Termination Notice
Number of LPCs signed LTA | Units	148
Percentage of total operating revenues	77.80%
Revenues	$ 10,636
Federal agencies and other [Member]
Regulated Operating Revenue	139	125	135
Local Power Company [Member]
Electric revenue	12,334
Regulated Operating Revenue	12,334	11,138	10,903
Industries Directly Served [Member]
Regulated Operating Revenue	$ 1,017	$ 868	$ 864