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SEGMENT AND RELATED INFORMATION - Narrative (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) state	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) state segment	Jun. 30, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of states in which entity operates \| state	3		3
Operating revenues	$ 4,765	$ 5,222	$ 10,053	$ 11,131
Operating revenues	4,620	5,098	9,638	10,510
Natural gas
Segment Reporting Information [Line Items]
Operating revenues	636	689	1,885	2,163
SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Operating revenues	268	315	519	592
Operating revenues	439	510	814	953
SOUTHERN POWER CO \| Wholesale revenues, affiliates
Segment Reporting Information [Line Items]
Operating revenues	92	117	178	204
SOUTHERN POWER CO \| Traditional Electric Operating Companies \| Wholesale revenues, affiliates
Segment Reporting Information [Line Items]
Operating revenues	$ 92	117	$ 178	204
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Number of states in which entity operates \| state	4		4
Operating revenues	$ 991	1,068	$ 2,556	3,180
Operating revenues	636	689	$ 1,885	2,163
Number of reportable segments \| segment			4
SOUTHERN Co GAS \| Natural gas
Segment Reporting Information [Line Items]
Operating revenues	638	688	$ 1,878	2,163
SOUTHERN Co GAS \| SOUTHERN POWER CO \| Natural gas
Segment Reporting Information [Line Items]
Operating revenues	$ 3	$ 16	$ 13	$ 33
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%