PROPERTY, PLANT, AND EQUIPMENT - Ownership and Investment in Jointly-Owned Facilities (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) mi
Alabama Power
Other Ownership Interests [Line Items]
Percent Ownership	14.00%
Alabama Power | Greene County (natural gas) Units 1 and 2
Other Ownership Interests [Line Items]
Percent Ownership	60.00%
Plant in Service	$ 189
Accumulated Depreciation	76
CWIP	$ 2
Alabama Power | Plant Miller (coal) Units 1 and 2
Other Ownership Interests [Line Items]
Percent Ownership	91.80%
Plant in Service	$ 2,107
Accumulated Depreciation	650
CWIP	$ 24
Georgia Power | Plant Hatch (nuclear)
Other Ownership Interests [Line Items]
Percent Ownership	50.10%
Plant in Service	$ 1,352
Accumulated Depreciation	624
CWIP	$ 37
Georgia Power | Plant Vogtle (nuclear) Units 1 and 2
Other Ownership Interests [Line Items]
Percent Ownership	45.70%
Plant in Service	$ 3,592
Accumulated Depreciation	2,221
CWIP	$ 55
Georgia Power | Plant Scherer (coal) Units 1 and 2
Other Ownership Interests [Line Items]
Percent Ownership	8.40%
Plant in Service	$ 279
Accumulated Depreciation	98
CWIP	$ 1
Georgia Power | Plant Scherer (coal) Unit 3
Other Ownership Interests [Line Items]
Percent Ownership	75.00%
Plant in Service	$ 1,320
Accumulated Depreciation	520
CWIP	$ 4
Georgia Power | Plant Wansley (coal)
Other Ownership Interests [Line Items]
Percent Ownership	53.50%
Plant in Service	$ 1,068
Accumulated Depreciation	418
CWIP	$ 10
Georgia Power | Rocky Mountain (pumped storage)
Other Ownership Interests [Line Items]
Percent Ownership	25.40%
Plant in Service	$ 183
Accumulated Depreciation	144
CWIP	$ 1
Mississippi Power | Plant Daniel (coal) Units 1 and 2
Other Ownership Interests [Line Items]
Percent Ownership	50.00%
Plant in Service	$ 775
Accumulated Depreciation	238
CWIP	$ 15
Southern Company Gas | Dalton Pipeline (natural gas pipeline)
Other Ownership Interests [Line Items]
Percent Ownership	50.00%
Plant in Service	$ 271
Accumulated Depreciation	15
CWIP	$ 0
Pipeline infrastructure (miles) | mi	115
Agreement to lease undivided ownership (percent)	50.00%
Future minimum payments receivable	$ 26
Alabama Power | Mississippi Power | Greene County (natural gas) Units 1 and 2
Other Ownership Interests [Line Items]
Percent Ownership	40.00%
Plant in Service	$ 122
Accumulated Depreciation	54
CWIP	$ 1