PROPERTY, PLANT, AND EQUIPMENT - Deferred Cloud Implementation Costs (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Deferred cloud implementation costs	$ 16,298	$ 15,469
Deferred Cloud Implementation Costs
Property, Plant and Equipment [Line Items]
Deferred cloud implementation costs	162
Alabama Power
Property, Plant and Equipment [Line Items]
Deferred cloud implementation costs	4,262	3,680
Alabama Power | Deferred Cloud Implementation Costs
Property, Plant and Equipment [Line Items]
Deferred cloud implementation costs	38
Georgia Power
Property, Plant and Equipment [Line Items]
Deferred cloud implementation costs	8,297	8,028
Georgia Power | Deferred Cloud Implementation Costs
Property, Plant and Equipment [Line Items]
Deferred cloud implementation costs	58
Mississippi Power
Property, Plant and Equipment [Line Items]
Deferred cloud implementation costs	805	775
Mississippi Power | Deferred Cloud Implementation Costs
Property, Plant and Equipment [Line Items]
Deferred cloud implementation costs	7
Southern Power
Property, Plant and Equipment [Line Items]
Deferred cloud implementation costs	1,150	1,870
Southern Power | Deferred Cloud Implementation Costs
Property, Plant and Equipment [Line Items]
Deferred cloud implementation costs	9
Southern Company Gas
Property, Plant and Equipment [Line Items]
Deferred cloud implementation costs	931	$ 918
Southern Company Gas | Deferred Cloud Implementation Costs
Property, Plant and Equipment [Line Items]
Deferred cloud implementation costs	$ 17