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Segment and Related Information - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) state	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) state segment	Sep. 30, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of states in which entity operates \| state	3		3
Operating revenues	$ 5,732	$ 5,755	$ 18,066	$ 15,771
SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Operating revenues	501	332	1,113	851
SOUTHERN POWER CO \| Traditional Electric Operating Companies \| Wholesale revenues, affiliates
Segment Reporting Information [Line Items]
Operating revenues	$ 167	101	$ 361	279
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Number of states in which entity operates \| state	4		4
Operating revenues	$ 605	950	$ 4,930	3,506
Number of reportable segments \| segment			4
SOUTHERN Co GAS \| SOUTHERN POWER CO \| Natural gas
Segment Reporting Information [Line Items]
Operating revenues		$ 9	$ 18	$ 22
Southern Natural Gas Company, LLC \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Ownership percentage, equity method investment	50.00%		50.00%
PennEast Pipeline \| 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%