Consolidated Entities and Equity Method Investments - Schedule of Earnings (Loss) from Equity Method Investments (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2021	Sep. 30, 2020
Schedule of Equity Method Investments [Line Items]
Earnings (Loss) from Equity Method Investments	$ 30		$ 33			$ 35	$ 105
SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Earnings (Loss) from Equity Method Investments	25		33			14	106
Asset impairment charges						84	0
SOUTHERN Co GAS | SNG
Schedule of Equity Method Investments [Line Items]
Earnings (Loss) from Equity Method Investments	27		30			93	95
SOUTHERN Co GAS | PennEast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings (Loss) from Equity Method Investments	(2)		2			(81)	5
Asset impairment charges	2			$ 84	$ 82	84
Asset impairment charges, net of tax	2	$ 67		$ 67	$ 58
SOUTHERN Co GAS | Other
Schedule of Equity Method Investments [Line Items]
Earnings (Loss) from Equity Method Investments	$ 0		$ 1			$ 2	$ 6