PROPERTY, PLANT, AND EQUIPMENT - Depreciation and Amortization (Narrative) (Details) - USD ($) $ in Millions		12 Months Ended
	Nov. 26, 2019	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Accumulated depreciation		$ 30,765	$ 31,038
Utility plant in service
Property, Plant and Equipment [Line Items]
Accumulated depreciation		30,000	30,300
Other plant in service
Property, Plant and Equipment [Line Items]
Accumulated depreciation		$ 732	766
Other plant in service | Maximum
Property, Plant and Equipment [Line Items]
Depreciation period of original cost		65 years
Mississippi Power
Property, Plant and Equipment [Line Items]
Base rate case filing, annual depreciation increase requested	$ 16
Accumulated depreciation		$ 1,463	1,429
Southern Company Gas
Property, Plant and Equipment [Line Items]
Accumulated depreciation		4,650	4,400
Southern Company Gas | Utility plant in service
Property, Plant and Equipment [Line Items]
Accumulated depreciation		4,500	4,300
Southern Company Gas | Other plant in service
Property, Plant and Equipment [Line Items]
Accumulated depreciation		$ 155	$ 129
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		15 years
Southern Company Gas | Storage facilities | Minimum
Property, Plant and Equipment [Line Items]
Depreciation period of original cost		40 years
Southern Company Gas | Storage facilities | Maximum
Property, Plant and Equipment [Line Items]
Depreciation period of original cost		60 years
Southern Company Gas | Other assets | Maximum
Property, Plant and Equipment [Line Items]
Depreciation period of original cost		65 years