DERIVATIVES - Cumulative Basis Adjustments for Fair Value Hedges (Details) - Fair Value Hedging - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Securities due within one year
Derivative Instruments, Gain (Loss) [Line Items]
Carrying Amount of the Hedged Item	$ 0	$ (1,509)
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item	0	(10)
Long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Carrying Amount of the Hedged Item	(3,280)	0
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item	9	0
Long-term debt | Southern Company Gas
Derivative Instruments, Gain (Loss) [Line Items]
Carrying Amount of the Hedged Item	(493)	0
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item	$ 2	$ 0