SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Affiliate Transactions (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Southern Nuclear Operating Company, Inc. | Alabama Power
Related Party Transaction [Line Items]
Costs for services		$ 256	$ 247	$ 248
Southern Nuclear Operating Company, Inc. | Georgia Power
Related Party Transaction [Line Items]
Costs for services		760	780	675
Southern Natural Gas Company, LLC | Alabama Power
Related Party Transaction [Line Items]
Costs for services		17	8	9
Southern Natural Gas Company, LLC | Georgia Power
Related Party Transaction [Line Items]
Costs for services		99	101	102
Southern Natural Gas Company, LLC | Southern Company Gas
Related Party Transaction [Line Items]
Costs for services		31	32	32
Southern Natural Gas Company, LLC | Southern Power
Related Party Transaction [Line Items]
Costs for services		28	25	25
Gulf Power | Alabama Power
Related Party Transaction [Line Items]
Amounts received related to tariff			11	11
Gulf Power | Georgia Power
Related Party Transaction [Line Items]
Costs for services			8	11
Gulf Power | Mississippi Power
Related Party Transaction [Line Items]
Costs for services			31	31
PowerSecure International, Inc. | Alabama Power
Related Party Transaction [Line Items]
Costs for services		7	24	11
Southern Power | Georgia Power
Related Party Transaction [Line Items]
Costs for services		177	216	235
Southern Company Services, Inc. | Alabama Power
Related Party Transaction [Line Items]
Costs for services		527	508	479
Southern Company Services, Inc. | Georgia Power
Related Party Transaction [Line Items]
Costs for services		704	653	625
Southern Company Services, Inc. | Southern Company Gas
Related Party Transaction [Line Items]
Costs for services		183	194	63
Southern Company Services, Inc. | Mississippi Power
Related Party Transaction [Line Items]
Costs for services		118	104	140
Southern Company Services, Inc. | Southern Power
Related Party Transaction [Line Items]
Costs for services		90	98	218
Non-Fuel Expense | Mississippi Power | Alabama Power
Related Party Transaction [Line Items]
Costs for services		9	8	9
Purchased Power from Affiliates | Southern Power
Related Party Transaction [Line Items]
Costs for services		174	215	233
Operating Lease PPA | Southern Power
Related Party Transaction [Line Items]
Costs for services		116	65	81
Electric Transmission | Southern Company Services, Inc. | Southern Power
Related Party Transaction [Line Items]
Costs for services		$ 15	$ 12	$ 13
Plant Scherer (coal) Unit 3 | Gulf Power | Georgia Power
Related Party Transaction [Line Items]
Proportionate share of related non-fuel expenses (percent)		25.00%
Subsequent Event | Southern Company Gas
Related Party Transaction [Line Items]
Proceeds from affiliate	$ 71
Subsequent Event | Southern Natural Gas Company, LLC | Georgia Power
Related Party Transaction [Line Items]
Proceeds from affiliate	$ 142