SEGMENT AND RELATED INFORMATION - Financial Data for Products and Services (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Concentration Risk [Line Items]
Operating revenues	$ 4,620	$ 5,098	$ 9,638	$ 10,510
Less Gross Gas Costs	144	191	583	877
Retail
Concentration Risk [Line Items]
Operating revenues	3,182	3,540	6,260	6,623
Wholesale
Concentration Risk [Line Items]
Operating revenues	472	542	889	1,041
Other
Concentration Risk [Line Items]
Operating revenues	168	161	320	331
Electric Utilities' Revenues
Concentration Risk [Line Items]
Operating revenues	3,884	4,290	7,579	8,085
Electric Utilities' Revenues | Retail
Concentration Risk [Line Items]
Operating revenues	3,182	3,540	6,260	6,623
Electric Utilities' Revenues | Wholesale
Concentration Risk [Line Items]
Operating revenues	472	542	889	1,041
Electric Utilities' Revenues | Other
Concentration Risk [Line Items]
Operating revenues	230	208	430	421
Southern Company Gas' Revenues
Concentration Risk [Line Items]
Operating revenues	636	689	1,885	2,163
Southern Company Gas' Revenues | Gas Distribution Operations
Concentration Risk [Line Items]
Operating revenues	583	563	1,596	1,724
Southern Company Gas' Revenues | Wholesale Gas Services
Concentration Risk [Line Items]
Operating revenues	(19)	48	32	134
Southern Company Gas' Revenues | Gas Marketing Services
Concentration Risk [Line Items]
Operating revenues	56	58	233	287
Southern Company Gas' Revenues | Other
Concentration Risk [Line Items]
Operating revenues	16	20	24	18
Southern Company Gas' Revenues
Concentration Risk [Line Items]
Operating revenues	636	689	1,885	2,163
Southern Company Gas' Revenues | Wholesale Gas Services
Concentration Risk [Line Items]
Operating revenues	(19)	48	32	134
Less Gross Gas Costs	891	1,238	2,054	3,165
Third Party Gross Revenues | Southern Company Gas' Revenues | Wholesale Gas Services
Concentration Risk [Line Items]
Operating revenues	854	1,223	2,039	3,148
Intercompany Revenues | Southern Company Gas' Revenues | Wholesale Gas Services
Concentration Risk [Line Items]
Operating revenues	18	63	47	151
Total Gross Revenues | Southern Company Gas' Revenues | Wholesale Gas Services
Concentration Risk [Line Items]
Operating revenues	$ 872	$ 1,286	$ 2,086	$ 3,299