RETIREMENT BENEFITS - Accumulated Benefit Obligation (Details) - Pension plans - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Total accumulated benefit obligation for the pension plans	$ 13,391	$ 11,683
Alabama Power
Defined Benefit Plan Disclosure [Line Items]
Total accumulated benefit obligation for the pension plans	3,053	2,550
Georgia Power
Defined Benefit Plan Disclosure [Line Items]
Total accumulated benefit obligation for the pension plans	4,222	3,613
Mississippi Power
Defined Benefit Plan Disclosure [Line Items]
Total accumulated benefit obligation for the pension plans	615	513
Southern Power
Defined Benefit Plan Disclosure [Line Items]
Total accumulated benefit obligation for the pension plans	151	101
Southern Company Gas
Defined Benefit Plan Disclosure [Line Items]
Total accumulated benefit obligation for the pension plans	$ 963	$ 842