RETIREMENT BENEFITS - Accumulated Benefit Obligation (Details) - Pension plans - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Total accumulated benefit obligation for the pension plans	$ 14,687	$ 14,922
Alabama Power
Defined Benefit Plan Disclosure [Line Items]
Total accumulated benefit obligation for the pension plans	3,362	3,414
Georgia Power
Defined Benefit Plan Disclosure [Line Items]
Total accumulated benefit obligation for the pension plans	4,562	4,657
Mississippi Power
Defined Benefit Plan Disclosure [Line Items]
Total accumulated benefit obligation for the pension plans	672	683
Southern Power
Defined Benefit Plan Disclosure [Line Items]
Total accumulated benefit obligation for the pension plans	178	175
Southern Company Gas
Defined Benefit Plan Disclosure [Line Items]
Total accumulated benefit obligation for the pension plans	$ 1,030	$ 1,072