Revenue Customer Type (Details) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2025 USD ($) Units	Mar. 31, 2024 USD ($)	Mar. 31, 2025 USD ($) Units	Mar. 31, 2024 USD ($)
Revenue, Major Customer [Line Items]
MLGW's % of operating revenues			700.00%	700.00%
NES's % of operating revenues			800.00%	800.00%
Off System Sales of Electricity	$ 1	$ 2	$ 2	$ 4
Bill credits for LTA	$ (61)	$ (54)	$ (111)	(101)
Number of LPCs signed LTA \| Units	148		148
Number of LPCs signed Flexibility Agreement \| Units	107		107
Percent of sales of electricity to LPCs		92.00%	92.00%
Pre-commercial Ops - capitalized revenue	$ (2)	$ 0	$ (2)	3
Regulated Operating Revenue	3,476	3,093	6,352	5,824
Unregulated Operating Revenue	56	61	100	95
Regulated and Unregulated Operating Revenue	3,532	3,154	6,452	5,919
lpcs [Domain]
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	3,199	2,842	5,815	5,329
industries directly served [Domain]
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	245	221	475	438
federal agencies and other [Domain]
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	34	30	64	60
ALABAMA
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	520	457	951	865
GEORGIA
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	94	80	168	149
KENTUCKY
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	214	201	396	378
MISSISSIPPI
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	307	284	576	539
NORTH CAROLINA
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	28	28	49	51
TENNESSEE
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	2,298	2,027	4,184	3,816
VIRGINIA
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	$ 16	$ 14	$ 28	$ 25