SEGMENT AND RELATED INFORMATION - Gas Revenues (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Total operating revenues	$ 20,375	$ 21,419	$ 23,495
Southern Company Gas
Segment Reporting Information [Line Items]
Total operating revenues	3,434	3,792	3,909
Operating Segments | Southern Company Gas
Segment Reporting Information [Line Items]
Total operating revenues	3,466	3,810	3,930
Southern Company Gas
Segment Reporting Information [Line Items]
Total operating revenues	3,434	3,792	3,909
Southern Company Gas | Gas distribution operations
Segment Reporting Information [Line Items]
Total operating revenues	2,902	3,001	3,155
Southern Company Gas | Gas marketing services
Segment Reporting Information [Line Items]
Total operating revenues	408	456	568
Southern Company Gas | Other Gas Revenue
Segment Reporting Information [Line Items]
Total operating revenues	124	335	186
Southern Company Gas | Operating Segments
Segment Reporting Information [Line Items]
Total operating revenues	3,434	3,792	3,909
Wholesale gas services | Operating Segments | Southern Company Gas
Segment Reporting Information [Line Items]
Total operating revenues	74	294	144
Less Gross Gas Costs	4,585	5,684	7,262
Wholesale gas services | Operating Segments | Third Party Gross Revenues | Southern Company Gas
Segment Reporting Information [Line Items]
Total operating revenues	4,544	5,703	6,955
Wholesale gas services | Operating Segments | Intercompany Revenues | Southern Company Gas
Segment Reporting Information [Line Items]
Total operating revenues	115	275	451
Wholesale gas services | Operating Segments | Total Gross Revenues | Southern Company Gas
Segment Reporting Information [Line Items]
Total operating revenues	$ 4,659	$ 5,978	$ 7,406