SEGMENT AND RELATED INFORMATION - Gas Revenues (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total operating revenues	$ 21,419	$ 23,495	$ 23,031
Southern Company Gas
Segment Reporting Information [Line Items]
Total operating revenues	3,792	3,909	3,920
Southern Company Gas | Gas Distribution Operations
Segment Reporting Information [Line Items]
Total operating revenues	3,001	3,155	3,024
Southern Company Gas | Gas marketing services
Segment Reporting Information [Line Items]
Total operating revenues	456	568	860
Southern Company Gas | Other Gas Revenue
Segment Reporting Information [Line Items]
Total operating revenues	$ 335	$ 186	$ 36