Revenue Customer Type (Details) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2024 USD ($) Units	Mar. 31, 2023 USD ($)	Mar. 31, 2024 USD ($) Units	Mar. 31, 2023 USD ($)
Revenue, Major Customer [Line Items]
MLGW's % of operating revenues			800.00%
NES's % of operating revenues			800.00%
Off System Sales of Electricity	$ 2		$ 4	$ 7
Revenues			5,919	5,974
Bill credits for LTA	$ (54)	$ (47)	$ (101)	(95)
Number of LPCs signed LTA \| Units	147		147
Number of LPCs signed Flexibility Agreement \| Units	94		94
Percent of sales of electricity to LPCs	92.00%
Pre-commercial Ops - capitalized revenue	$ 0	0	$ (3)	0
Regulated Operating Revenue	3,093	2,924	5,824	5,887
Regulated and Unregulated Operating Revenue	3,154	2,959	5,919	5,974
lpcs [Domain]
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	2,842	2,674	5,329	5,380
industries directly served [Domain]
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	221	217	438	442
federal agencies and other [Domain]
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	30	33	60	65
ALABAMA
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	457	428	865	856
GEORGIA
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	80	72	149	148
KENTUCKY
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	201	192	378	390
MISSISSIPPI
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	284	273	539	551
NORTH CAROLINA
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	28	25	51	50
TENNESSEE
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	2,027	1,918	3,816	3,859
VIRGINIA
Revenue, Major Customer [Line Items]
Regulated Operating Revenue	$ 14	$ 13	$ 25	$ 26