PROPERTY, PLANT, AND EQUIPMENT - Depreciation and Amortization (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Accumulated depreciation	$ 34,079	$ 32,397
Utility plant in service
Property, Plant and Equipment [Line Items]
Accumulated depreciation	33,100	31,600
Other plant in service
Property, Plant and Equipment [Line Items]
Accumulated depreciation	$ 930	817
Other plant in service | Maximum
Property, Plant and Equipment [Line Items]
Depreciation period of original cost	37 years
Southern Company Gas
Property, Plant and Equipment [Line Items]
Accumulated depreciation	$ 5,067	4,821
Southern Company Gas | Utility plant in service
Property, Plant and Equipment [Line Items]
Accumulated depreciation	4,800	4,600
Southern Company Gas | Other plant in service
Property, Plant and Equipment [Line Items]
Accumulated depreciation	$ 219	$ 195
Southern Company Gas | Transportation equipment | Minimum
Property, Plant and Equipment [Line Items]
Depreciation period of original cost	5 years
Southern Company Gas | Transportation equipment | Maximum
Property, Plant and Equipment [Line Items]
Depreciation period of original cost	12 years
Southern Company Gas | Storage facilities | Minimum
Property, Plant and Equipment [Line Items]
Depreciation period of original cost	30 years
Southern Company Gas | Storage facilities | Maximum
Property, Plant and Equipment [Line Items]
Depreciation period of original cost	75 years
Southern Company Gas | Other assets | Maximum
Property, Plant and Equipment [Line Items]
Depreciation period of original cost	75 years