CONSOLIDATED ENTITIES AND EQUITY METHOD INVESTMENTS - Income Statement Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	$ 162	$ 148	$ 106
Southern Company Gas
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	157	148	106
Southern Company Gas | SNG
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	141	131	88
Revenues	630	604	544
Operating income	335	310	242
Net income	280	261	175
Southern Company Gas | Atlantic Coast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	13	7	6
Southern Company Gas | PennEast Pipeline(a)
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	6	5	6
Southern Company Gas | Other
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	$ (3)	$ 5	$ 6