PROPERTY, PLANT, AND EQUIPMENT - Composite Straight-Line Rates (Details) - Utility plant in service	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Alabama Power
Property, Plant and Equipment [Line Items]
Composite rate	3.10%	3.00%	2.90%
Georgia Power
Property, Plant and Equipment [Line Items]
Composite rate	2.60%	2.60%	2.70%
Mississippi Power
Property, Plant and Equipment [Line Items]
Composite rate	3.70%	4.20%	3.40%
Southern Company Gas
Property, Plant and Equipment [Line Items]
Composite rate	2.90%	2.90%	2.90%