SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Costs for Affiliate Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Alabama Power | Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	$ 527	$ 508	$ 479
Alabama Power | Southern Natural Gas Company, LLC
Related Party Transaction [Line Items]
Costs for services	17	8	9
Georgia Power | Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	704	653	625
Georgia Power | Southern Natural Gas Company, LLC
Related Party Transaction [Line Items]
Costs for services	99	101	102
Georgia Power | Southern Company Gas
Related Party Transaction [Line Items]
Costs for services	4	21	22
Mississippi Power | Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	118	104	140
Mississippi Power | Alabama Power | Power Pool
Related Party Transaction [Line Items]
Costs for services	3	15	16
Southern Power | Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	90	98	218
Southern Power | Alabama Power | Power Pool
Related Party Transaction [Line Items]
Costs for services	14	41	27
Southern Power | Southern Natural Gas Company, LLC
Related Party Transaction [Line Items]
Costs for services	28	25	25
Southern Power | Southern Company Gas
Related Party Transaction [Line Items]
Costs for services	64	119	119
Southern Company Gas | Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	183	194	63
Southern Company Gas | Southern Natural Gas Company, LLC
Related Party Transaction [Line Items]
Costs for services	$ 31	$ 32	$ 32