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SEGMENT AND RELATED INFORMATION - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) state	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment state	Sep. 30, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of states in which entity operates \| state	3		3
Operating revenues	$ 5,755	$ 6,282	$ 15,771	$ 17,415
SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Operating revenues	332	372	851	980
SOUTHERN POWER CO \| Traditional Electric Operating Companies \| Wholesale revenues, affiliates
Segment Reporting Information [Line Items]
Operating revenues	$ 101	116	$ 279	320
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Number of states in which entity operates \| state	4		4
Operating revenues	$ 950	913	$ 3,506	4,093
Number of reportable segments \| segment			4
SOUTHERN Co GAS \| Traditional Electric Operating Companies \| Natural gas
Segment Reporting Information [Line Items]
Operating revenues		9		13
SOUTHERN Co GAS \| SOUTHERN POWER CO \| Natural gas
Segment Reporting Information [Line Items]
Operating revenues	$ 9	$ 20	$ 22	$ 53
Southern Natural Gas Company, LLC \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Ownership percentage, equity method investment	50.00%		50.00%
PennEast Pipelines \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Ownership percentage, equity method investment	20.00%		20.00%
Dalton Pipeline \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Ownership percentage, equity method investment	50.00%		50.00%
Atlantic Coast Pipeline \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Ownership percentage, equity method investment	5.00%		5.00%