DERIVATIVES - Cumulative Basis Adjustments for Fair Value Hedges (Details) - Long-term debt - Fair Value Hedging - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Derivative Instruments, Gain (Loss) [Line Items]
Carrying Amount of the Hedged Item	$ (3,024)	$ (2,927)
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item	235	282
Southern Company Gas
Derivative Instruments, Gain (Loss) [Line Items]
Carrying Amount of the Hedged Item	(427)	(415)
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item	$ 70	$ 81