SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Costs for Affiliate Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Alabama Power | Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	$ 478	$ 527	$ 508
Alabama Power | Southern Natural Gas Company, LLC
Related Party Transaction [Line Items]
Costs for services	15	17	8
Georgia Power | Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	639	704	653
Georgia Power | Southern Natural Gas Company, LLC
Related Party Transaction [Line Items]
Costs for services	108	99	101
Georgia Power | Southern Company Gas
Related Party Transaction [Line Items]
Costs for services	0	4	21
Mississippi Power | Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	149	118	104
Mississippi Power | Alabama Power | Power Pool
Related Party Transaction [Line Items]
Costs for services	4	3	15
Southern Power | Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	87	90	98
Southern Power | Alabama Power | Power Pool
Related Party Transaction [Line Items]
Costs for services	8	14	41
Southern Power | Southern Natural Gas Company, LLC
Related Party Transaction [Line Items]
Costs for services	29	28	25
Southern Power | Southern Company Gas
Related Party Transaction [Line Items]
Costs for services	26	64	119
Southern Company Gas | Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	237	183	194
Southern Company Gas | Southern Natural Gas Company, LLC
Related Party Transaction [Line Items]
Costs for services	$ 29	$ 31	$ 32