SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Affiliate Transactions (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Southern Nuclear Operating Company, Inc. | Alabama Power
Related Party Transaction [Line Items]
Costs for services	$ 251	$ 267	$ 258
Southern Nuclear Operating Company, Inc. | Georgia Power
Related Party Transaction [Line Items]
Costs for services	899	895	906
Southern Power | Georgia Power
Related Party Transaction [Line Items]
Costs for services	143	151	132
Southern Natural Gas Company, LLC | Alabama Power
Related Party Transaction [Line Items]
Costs for services	12	18	14
Southern Natural Gas Company, LLC | Georgia Power
Related Party Transaction [Line Items]
Costs for services	101	99	108
Southern Natural Gas Company, LLC | Southern Power
Related Party Transaction [Line Items]
Costs for services	34	37	31
Southern Natural Gas Company, LLC | Southern Company Gas
Related Party Transaction [Line Items]
Costs for services	28	27	29
Southern Company Gas | Southern Power
Related Party Transaction [Line Items]
Costs for services	0	0	18
Southern Company Services, Inc. | Alabama Power
Related Party Transaction [Line Items]
Costs for services	611	549	504
Southern Company Services, Inc. | Georgia Power
Related Party Transaction [Line Items]
Costs for services	857	762	663
Southern Company Services, Inc. | Southern Power
Related Party Transaction [Line Items]
Costs for services	86	86	89
Southern Company Services, Inc. | Southern Company Gas
Related Party Transaction [Line Items]
Costs for services	261	262	239
PowerSecure | Southern Company Gas | US General Services Administration
Related Party Transaction [Line Items]
Costs for services	29	10
Contract amount		77
Non-Fuel Expense | Mississippi Power | Alabama Power
Related Party Transaction [Line Items]
Costs for services	5	6	10
Purchased Power from Affiliates | Southern Power
Related Party Transaction [Line Items]
Costs for services	145	154	139
Operating Lease PPA | Southern Power
Related Party Transaction [Line Items]
Costs for services	116	116	112
Electric Transmission | Southern Company Services, Inc. | Southern Power
Related Party Transaction [Line Items]
Costs for services	$ 33	$ 39	$ 28