SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Goodwill Balances (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Goodwill [Line Items]
Goodwill	$ 5,161	$ 5,161	$ 5,280
Southern Company Gas
Goodwill [Line Items]
Goodwill	5,015	5,015
Gas distribution operations | Southern Company Gas
Goodwill [Line Items]
Goodwill	4,034	4,034
Gas marketing services | Southern Company Gas
Goodwill [Line Items]
Goodwill	$ 981	$ 981