RETIREMENT BENEFITS - 401K Plan Matching Contributions (Details) - Employee Savings Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost	$ 113	$ 119	$ 118
Alabama Power
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost	25	24	23
Georgia Power
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost	27	26	26
Mississippi Power
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost	4	5	5
Southern Power
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost	2	3
Southern Company Gas
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost	$ 15	$ 18	$ 19