REGULATORY MATTERS - Cost Recovery Clauses (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
ALABAMA POWER CO | Other regulatory liabilities, current
Loss Contingencies [Line Items]
Rate CNP Compliance	$ 25	$ 55
Retail Energy Cost Recovery	22	32
Natural Disaster Reserve	16	37
ALABAMA POWER CO | Other regulatory liabilities, deferred
Loss Contingencies [Line Items]
Rate CNP Compliance	0	7
Retail Energy Cost Recovery	93	17
Natural Disaster Reserve	96	113
ALABAMA POWER CO | Deferred under recovered regulatory clause revenues
Loss Contingencies [Line Items]
Rate CNP PPA	41	40
GEORGIA POWER CO | Other current liabilities
Loss Contingencies [Line Items]
Fuel Cost Recovery	109	0
GEORGIA POWER CO | Other deferred credits and liabilities
Loss Contingencies [Line Items]
Fuel Cost Recovery	95	73
MISSISSIPPI POWER CO | Other regulatory liabilities, deferred
Loss Contingencies [Line Items]
Property Damage Reserve	50	54
MISSISSIPPI POWER CO | Over recovered regulatory clause liabilities
Loss Contingencies [Line Items]
Fuel Cost Recovery	22	23
MISSISSIPPI POWER CO | Other regulatory assets
Loss Contingencies [Line Items]
Ad Valorem Tax	11	47
MISSISSIPPI POWER CO | Other regulatory assets, deferred
Loss Contingencies [Line Items]
Ad Valorem Tax	39	0
SOUTHERN Co GAS | Other regulatory liabilities
Loss Contingencies [Line Items]
Natural Gas Cost Recovery	$ 100	$ 74