CONSOLIDATED ENTITIES AND EQUITY METHOD INVESTMENTS - Balance Sheet Equity Method Investment Carrying Amount (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Investment Balance	$ 1,358	$ 1,303
SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Investment Balance	1,301	1,251
SOUTHERN Co GAS | SNG
Schedule of Equity Method Investments [Line Items]
Investment Balance	1,180	1,137
SOUTHERN Co GAS | PennEast Pipeline
Schedule of Equity Method Investments [Line Items]
Investment Balance	89	82
SOUTHERN Co GAS | Other
Schedule of Equity Method Investments [Line Items]
Investment Balance	$ 32	$ 32