SEGMENT AND RELATED INFORMATION - Financial Data for Products and Services (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Concentration Risk [Line Items]
Operating revenues	$ 5,620	$ 5,995	$ 15,258	$ 16,505
Less Gross Gas Costs	71	79	654	956
Impairment charges	0	110	0	142
Retail
Concentration Risk [Line Items]
Operating revenues	4,243	4,512	10,503	11,136
Wholesale
Concentration Risk [Line Items]
Operating revenues	584	625	1,473	1,667
Other
Concentration Risk [Line Items]
Operating revenues	164	163	484	492
Electric Utilities' Revenues
Concentration Risk [Line Items]
Operating revenues	5,049	5,363	12,628	13,448
Electric Utilities' Revenues \| Retail
Concentration Risk [Line Items]
Operating revenues	4,243	4,512	10,503	11,136
Electric Utilities' Revenues \| Wholesale
Concentration Risk [Line Items]
Operating revenues	584	625	1,473	1,667
Electric Utilities' Revenues \| Other
Concentration Risk [Line Items]
Operating revenues	222	226	652	645
Southern Company Gas' Revenues
Concentration Risk [Line Items]
Operating revenues	477	498	2,362	2,661
Southern Company Gas' Revenues \| Gas Distribution Operations
Concentration Risk [Line Items]
Operating revenues	476	445	2,072	2,169
Southern Company Gas' Revenues \| Wholesale Gas Services
Concentration Risk [Line Items]
Operating revenues	(51)	(2)	(19)	132
Southern Company Gas' Revenues \| Gas Marketing Services
Concentration Risk [Line Items]
Operating revenues	39	39	272	326
Southern Company Gas' Revenues \| Other
Concentration Risk [Line Items]
Operating revenues	13	16	37	34
Southern Company Gas' Revenues
Concentration Risk [Line Items]
Operating revenues	477	498	2,362	2,661
Impairment charges	0	92	0	92
Impairment charges, net of tax		65		65
Southern Company Gas' Revenues \| Wholesale Gas Services
Concentration Risk [Line Items]
Operating revenues	(51)	(2)	(19)	132
Less Gross Gas Costs	1,134	1,212	3,189	4,378
Third Party Gross Revenues \| Southern Company Gas' Revenues \| Wholesale Gas Services
Concentration Risk [Line Items]
Operating revenues	1,050	1,138	3,089	4,287
Intercompany Revenues \| Southern Company Gas' Revenues \| Wholesale Gas Services
Concentration Risk [Line Items]
Operating revenues	33	72	81	223
Total Gross Revenues \| Southern Company Gas' Revenues \| Wholesale Gas Services
Concentration Risk [Line Items]
Operating revenues	$ 1,083	$ 1,210	$ 3,170	$ 4,510

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