Revenue Customer Type (Details) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2025 USD ($) Units	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($) Units	Jun. 30, 2024 USD ($)
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	$ 4	$ 6
Bill credits for LTA	$ (53)	$ (51)	$ (164)	$ (152)
Number of LPCs signed LTA \| Units	148		148
Number of LPCs signed Flexibility Agreement \| Units	107		107
Percent of sales of electricity to LPCs	91.00%	92.00%	91.00%	92.00%
Pre-commercial Ops - capitalized revenue	$ (1)	$ 0	$ (3)	$ 3
Regulated Operating Revenue	3,261	2,830	9,613	8,654
Unregulated Operating Revenue	45	49	145	144
Regulated and Unregulated Operating Revenue	3,306	2,879	9,758	8,798
lpcs [Domain]
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	2,967	2,596	8,782	7,925
industries directly served [Domain]
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	259	205	734	642
federal agencies and other [Domain]
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	36	29	100	90
ALABAMA
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	477	409	1,428	1,274
GEORGIA
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	74	65	242	214
KENTUCKY
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	209	179	605	557
MISSISSIPPI
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	310	277	886	816
NORTH CAROLINA
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	18	19	67	70
TENNESSEE
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	2,161	1,869	6,345	5,685
VIRGINIA
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	$ 11	$ 10	$ 39	$ 35