SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Affiliate Transactions (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Southern Company Gas
Related Party Transaction [Line Items]
Proceeds from affiliate		$ 71,000,000
Southern Nuclear Operating Company, Inc. | Alabama Power
Related Party Transaction [Line Items]
Costs for services	$ 258,000,000	262,000,000	$ 256,000,000
Southern Nuclear Operating Company, Inc. | Georgia Power
Related Party Transaction [Line Items]
Costs for services	906,000,000	883,000,000	760,000,000
Southern Power | Georgia Power
Related Party Transaction [Line Items]
Costs for services	132,000,000	141,000,000	177,000,000
Southern Natural Gas Company, LLC | Alabama Power
Related Party Transaction [Line Items]
Costs for services	14,000,000	15,000,000	17,000,000
Southern Natural Gas Company, LLC | Georgia Power
Related Party Transaction [Line Items]
Costs for services	108,000,000	108,000,000	99,000,000
Proceeds from affiliate		142,000,000
Southern Natural Gas Company, LLC | Southern Power
Related Party Transaction [Line Items]
Costs for services	31,000,000	29,000,000	28,000,000
Southern Natural Gas Company, LLC | Southern Company Gas
Related Party Transaction [Line Items]
Costs for services	29,000,000	29,000,000	31,000,000
Southern Company Gas | Southern Power
Related Party Transaction [Line Items]
Costs for services	18,000,000	26,000,000	64,000,000
PowerSecure International, Inc. | Alabama Power
Related Party Transaction [Line Items]
Costs for services	0
PowerSecure International, Inc. | Georgia Power
Related Party Transaction [Line Items]
Costs for services		0	0
Southern Company Services, Inc. | Alabama Power
Related Party Transaction [Line Items]
Costs for services	504,000,000	478,000,000	527,000,000
Southern Company Services, Inc. | Georgia Power
Related Party Transaction [Line Items]
Costs for services	663,000,000	639,000,000	704,000,000
Southern Company Services, Inc. | Southern Power
Related Party Transaction [Line Items]
Costs for services	89,000,000	87,000,000	90,000,000
Southern Company Services, Inc. | Southern Company Gas
Related Party Transaction [Line Items]
Costs for services	239,000,000	237,000,000	183,000,000
Non-Fuel Expense | Mississippi Power | Alabama Power
Related Party Transaction [Line Items]
Costs for services	10,000,000	9,000,000	9,000,000
Purchased Power from Affiliates | Southern Power
Related Party Transaction [Line Items]
Costs for services	139,000,000	139,000,000	174,000,000
Operating Lease PPA | Southern Power
Related Party Transaction [Line Items]
Costs for services	112,000,000	115,000,000	116,000,000
Electric Transmission | Southern Company Services, Inc. | Southern Power
Related Party Transaction [Line Items]
Costs for services	$ 28,000,000	$ 15,000,000	$ 15,000,000