REGULATORY MATTERS - Cost Recovery Clauses (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
ALABAMA POWER CO | Other regulatory liabilities, current
Loss Contingencies [Line Items]
Rate CNP Compliance	$ 35	$ 55
Retail Energy Cost Recovery	74	32
Natural Disaster Reserve	27	37
ALABAMA POWER CO | Other regulatory liabilities, deferred
Loss Contingencies [Line Items]
Rate CNP Compliance	17	7
Retail Energy Cost Recovery	22	17
Natural Disaster Reserve	104	113
ALABAMA POWER CO | Deferred under recovered regulatory clause revenues
Loss Contingencies [Line Items]
Rate CNP PPA	37	40
GEORGIA POWER CO | Other current liabilities
Loss Contingencies [Line Items]
Fuel Cost Recovery	6	0
GEORGIA POWER CO | Other deferred credits and liabilities
Loss Contingencies [Line Items]
Fuel Cost Recovery	163	73
MISSISSIPPI POWER CO | Other regulatory liabilities, deferred
Loss Contingencies [Line Items]
Property Damage Reserve	53	54
MISSISSIPPI POWER CO | Over recovered regulatory clause liabilities
Loss Contingencies [Line Items]
Fuel Cost Recovery	25	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	38	0
SOUTHERN Co GAS | Other regulatory liabilities
Loss Contingencies [Line Items]
Natural Gas Cost Recovery	$ 84	$ 74