SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Affiliate Transactions (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Southern Nuclear Operating Company, Inc. | Alabama Power
Related Party Transaction [Line Items]
Costs for services	$ 267	$ 258	$ 262
Southern Nuclear Operating Company, Inc. | Georgia Power
Related Party Transaction [Line Items]
Costs for services	895	906	883
Southern Power | Georgia Power
Related Party Transaction [Line Items]
Costs for services	151	132	141
Southern Natural Gas Company, LLC | Alabama Power
Related Party Transaction [Line Items]
Costs for services	18	14	15
Southern Natural Gas Company, LLC | Georgia Power
Related Party Transaction [Line Items]
Costs for services	99	108	108
Southern Natural Gas Company, LLC | Southern Power
Related Party Transaction [Line Items]
Costs for services	37	31	29
Southern Natural Gas Company, LLC | Southern Company Gas
Related Party Transaction [Line Items]
Costs for services	27	29	29
Southern Company Gas | Southern Power
Related Party Transaction [Line Items]
Costs for services	0	18	26
Southern Company Services, Inc. | Alabama Power
Related Party Transaction [Line Items]
Costs for services	549	504	478
Southern Company Services, Inc. | Georgia Power
Related Party Transaction [Line Items]
Costs for services	762	663	639
Southern Company Services, Inc. | Southern Power
Related Party Transaction [Line Items]
Costs for services	86	89	87
Southern Company Services, Inc. | Southern Company Gas
Related Party Transaction [Line Items]
Costs for services	262	239	237
PowerSecure | Southern Company Gas | US General Services Administration
Related Party Transaction [Line Items]
Costs for services	10
Contract amount	70
Non-Fuel Expense | Mississippi Power | Alabama Power
Related Party Transaction [Line Items]
Costs for services	6	10	9
Purchased Power from Affiliates | Southern Power
Related Party Transaction [Line Items]
Costs for services	154	139	139
Operating Lease PPA | Southern Power
Related Party Transaction [Line Items]
Costs for services	116	112	115
Electric Transmission | Southern Company Services, Inc. | Southern Power
Related Party Transaction [Line Items]
Costs for services	$ 39	$ 28	$ 15