RETIREMENT BENEFITS - 401K Plan Matching Contributions (Details) - Employee Savings Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost	$ 119	$ 120	$ 113
Alabama Power
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost	26	26	25
Georgia Power
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost	28	29	27
Mississippi Power
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost	5	5	4
Southern Power
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost	2	2	2
Southern Company Gas
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost	$ 16	$ 16	$ 15