RETIREMENT BENEFITS - Schedule of Voluntarily Contributions (Details) - Pension plans - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Contributions to qualified pension plan	$ 1,208	$ 55
Alabama Power
Defined Benefit Plan Disclosure [Line Items]
Contributions to qualified pension plan	383	13
Georgia Power
Defined Benefit Plan Disclosure [Line Items]
Contributions to qualified pension plan	243	14
Mississippi Power
Defined Benefit Plan Disclosure [Line Items]
Contributions to qualified pension plan	59	3
Southern Power
Defined Benefit Plan Disclosure [Line Items]
Contributions to qualified pension plan	7	0
Southern Company Gas
Defined Benefit Plan Disclosure [Line Items]
Contributions to qualified pension plan	144	$ 2
Qualified pension plan
Defined Benefit Plan Disclosure [Line Items]
Contributions to qualified pension plan	1,136
Qualified pension plan | Alabama Power
Defined Benefit Plan Disclosure [Line Items]
Contributions to qualified pension plan	362
Qualified pension plan | Georgia Power
Defined Benefit Plan Disclosure [Line Items]
Contributions to qualified pension plan	200
Qualified pension plan | Mississippi Power
Defined Benefit Plan Disclosure [Line Items]
Contributions to qualified pension plan	54
Qualified pension plan | Southern Power
Defined Benefit Plan Disclosure [Line Items]
Contributions to qualified pension plan	24
Qualified pension plan | Southern Company Gas
Defined Benefit Plan Disclosure [Line Items]
Contributions to qualified pension plan	$ 145