SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Costs for Affiliate Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Alabama Power | Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	$ 504	$ 478	$ 527
Alabama Power | Southern Natural Gas Company, LLC
Related Party Transaction [Line Items]
Costs for services	14	15	17
Georgia Power | Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	663	639	704
Georgia Power | Southern Natural Gas Company, LLC
Related Party Transaction [Line Items]
Costs for services	108	108	99
Mississippi Power | Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	120	149	118
Mississippi Power | Alabama Power | Power Pool
Related Party Transaction [Line Items]
Costs for services	9	4	3
Southern Power | Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	89	87	90
Southern Power | Alabama Power | Power Pool
Related Party Transaction [Line Items]
Costs for services	15	8	14
Southern Power | Southern Natural Gas Company, LLC
Related Party Transaction [Line Items]
Costs for services	31	29	28
Southern Company Gas | Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	239	237	183
Southern Company Gas | Southern Natural Gas Company, LLC
Related Party Transaction [Line Items]
Costs for services	$ 29	$ 29	$ 31