Consolidated Entities and Equity Method Investments - Income Statement Equity Method Investment Carrying Amount (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Schedule of Equity Method Investments [Line Items]
Earnings from Equity Method Investments	$ 39	$ 36	$ 120	$ 108
SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Earnings from Equity Method Investments	35	34	115	108
SOUTHERN Co GAS | SNG(a)
Schedule of Equity Method Investments [Line Items]
Earnings from Equity Method Investments	30	29	104	95
SOUTHERN Co GAS | Atlantic Coast Pipeline(b)
Schedule of Equity Method Investments [Line Items]
Earnings from Equity Method Investments	3	1	9	4
SOUTHERN Co GAS | PennEast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from Equity Method Investments	2	2	5	4
SOUTHERN Co GAS | Other
Schedule of Equity Method Investments [Line Items]
Earnings from Equity Method Investments	$ 0	$ 2	$ (3)	$ 5