PROPERTY, PLANT, AND EQUIPMENT - Composite Straight-Line Rates (Details) - Utility plant in service	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Alabama Power
Property, Plant and Equipment [Line Items]
Composite rate	2.70%	2.60%	3.10%
Georgia Power
Property, Plant and Equipment [Line Items]
Composite rate	3.30%	3.00%	2.60%
Mississippi Power
Property, Plant and Equipment [Line Items]
Composite rate	3.60%	3.70%	3.70%
Southern Company Gas
Property, Plant and Equipment [Line Items]
Composite rate	2.80%	2.80%	2.90%