Revenue Customer Type (Details) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2024 USD ($) Units	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($) Units	Jun. 30, 2023 USD ($)
Revenue, Major Customer [Line Items]
MLGW's % of operating revenues			800.00%	800.00%
NES's % of operating revenues			800.00%	800.00%
Off System Sales of Electricity	$ 2	$ 2	$ 6	$ 9
Bill credits for LTA	$ (51)	(45)	$ (152)	(140)
Number of LPCs signed LTA \| Units	147		147
Number of LPCs signed Flexibility Agreement \| Units	97		97
Percent of sales of electricity to LPCs	92.00%		92.00%
Pre-commercial Ops - capitalized revenue	$ 0	0	$ (3)	0
Regulated Operating Revenue	2,830	2,664	8,654	8,551
Unregulated Operating Revenue	49	34	144	121
Regulated and Unregulated Operating Revenue	2,879	2,698	8,798	8,672
lpcs [Domain]
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	2,596	2,431	7,925	7,811
industries directly served [Domain]
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	205	203	642	645
federal agencies and other [Domain]
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	29	30	90	95
ALABAMA
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	409	388	1,274	1,244
GEORGIA
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	65	59	214	207
KENTUCKY
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	179	172	557	562
MISSISSIPPI
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	277	263	816	814
NORTH CAROLINA
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	19	17	70	67
TENNESSEE
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	1,869	1,754	5,685	5,613
VIRGINIA
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	$ 10	$ 9	$ 35	$ 35