REVENUE FROM CONTRACTS WITH CUSTOMERS AND LEASE INCOME - Disaggregate Revenue Sources (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Operating revenues	$ 5,755	$ 6,282	$ 15,771	$ 17,415
Revenues	5,620	5,995	15,258	16,505
Total retail electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	4,178	4,661	10,540	11,387
Residential
Disaggregation of Revenue [Line Items]
Operating revenues	2,019	2,119	4,802	4,939
Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	1,354	1,563	3,589	3,951
Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	783	953	2,081	2,428
Other
Disaggregation of Revenue [Line Items]
Operating revenues	22	26	68	69
Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Operating revenues	474	441	2,058	2,158
Residential
Disaggregation of Revenue [Line Items]
Operating revenues	170	162	906	992
Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	41	42	229	277
Transportation
Disaggregation of Revenue [Line Items]
Operating revenues	224	204	723	673
Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	4	3	21	25
Other
Disaggregation of Revenue [Line Items]
Operating revenues	35	30	179	191
Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	409	441	1,048	1,172
PPA energy revenues
Disaggregation of Revenue [Line Items]
Operating revenues	214	245	550	648
PPA capacity revenues
Disaggregation of Revenue [Line Items]
Operating revenues	136	134	339	350
Non-PPA revenues
Disaggregation of Revenue [Line Items]
Operating revenues	59	62	159	174
Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	476	472	1,448	1,935
Wholesale gas services
Disaggregation of Revenue [Line Items]
Operating revenues	431	425	1,168	1,590
Gas marketing services
Disaggregation of Revenue [Line Items]
Operating revenues	38	37	258	313
Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	7	10	22	32
Other revenues
Disaggregation of Revenue [Line Items]
Operating revenues	218	267	677	763
Other revenue sources
Disaggregation of Revenue [Line Items]
Other	968	855	2,604	3,266
Other adjustments
Disaggregation of Revenue [Line Items]
Other	(1,103)	(1,142)	(3,117)	(4,176)
ALABAMA POWER CO
Disaggregation of Revenue [Line Items]
Operating revenues	1,671	1,866	4,285	4,791
Revenues	1,729	1,841	4,445	4,762
ALABAMA POWER CO | Total retail electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	1,562	1,760	3,963	4,420
ALABAMA POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	752	820	1,839	1,971
ALABAMA POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	447	512	1,152	1,301
ALABAMA POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	358	421	956	1,128
ALABAMA POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	5	7	16	20
ALABAMA POWER CO | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
ALABAMA POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
ALABAMA POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
ALABAMA POWER CO | Transportation
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
ALABAMA POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
ALABAMA POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
ALABAMA POWER CO | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	76	70	205	252
ALABAMA POWER CO | PPA energy revenues
Disaggregation of Revenue [Line Items]
Operating revenues	40	43	94	110
ALABAMA POWER CO | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Operating revenues	26	25	78	77
ALABAMA POWER CO | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Operating revenues	10	2	33	65
ALABAMA POWER CO | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
ALABAMA POWER CO | Wholesale gas services
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
ALABAMA POWER CO | Gas marketing services
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
ALABAMA POWER CO | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
ALABAMA POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Operating revenues	33	36	117	119
ALABAMA POWER CO | Other revenue sources
Disaggregation of Revenue [Line Items]
Other	58	(25)	160	(29)
ALABAMA POWER CO | Other adjustments
Disaggregation of Revenue [Line Items]
Other	0	0	0	0
GEORGIA POWER CO
Disaggregation of Revenue [Line Items]
Operating revenues	2,529	2,790	6,359	6,684
Revenues	2,617	2,755	6,371	6,706
GEORGIA POWER CO | Total retail electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	2,383	2,642	5,955	6,293
GEORGIA POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	1,183	1,211	2,760	2,751
GEORGIA POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	833	965	2,242	2,427
GEORGIA POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	352	450	907	1,074
GEORGIA POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	15	16	46	41
GEORGIA POWER CO | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
GEORGIA POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
GEORGIA POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
GEORGIA POWER CO | Transportation
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
GEORGIA POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
GEORGIA POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
GEORGIA POWER CO | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	31	35	75	93
GEORGIA POWER CO | PPA energy revenues
Disaggregation of Revenue [Line Items]
Operating revenues	13	19	38	47
GEORGIA POWER CO | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Operating revenues	15	13	30	41
GEORGIA POWER CO | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Operating revenues	3	3	7	5
GEORGIA POWER CO | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
GEORGIA POWER CO | Wholesale gas services
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
GEORGIA POWER CO | Gas marketing services
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
GEORGIA POWER CO | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
GEORGIA POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Operating revenues	115	113	329	298
GEORGIA POWER CO | Other revenue sources
Disaggregation of Revenue [Line Items]
Other	88	(35)	12	22
GEORGIA POWER CO | Other adjustments
Disaggregation of Revenue [Line Items]
Other	0	0	0	0
MISSISSIPPI POWER CO
Disaggregation of Revenue [Line Items]
Operating revenues	335	377	886	973
Revenues	336	370	895	970
MISSISSIPPI POWER CO | Total retail electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	233	259	622	674
MISSISSIPPI POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	84	88	203	217
MISSISSIPPI POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	74	86	195	223
MISSISSIPPI POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	73	82	218	226
MISSISSIPPI POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	2	3	6	8
MISSISSIPPI POWER CO | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
MISSISSIPPI POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
MISSISSIPPI POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
MISSISSIPPI POWER CO | Transportation
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
MISSISSIPPI POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
MISSISSIPPI POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
MISSISSIPPI POWER CO | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	96	114	245	285
MISSISSIPPI POWER CO | PPA energy revenues
Disaggregation of Revenue [Line Items]
Operating revenues	2	2	7	8
MISSISSIPPI POWER CO | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Operating revenues	1	1	3	2
MISSISSIPPI POWER CO | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Operating revenues	93	111	235	275
MISSISSIPPI POWER CO | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
MISSISSIPPI POWER CO | Wholesale gas services
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
MISSISSIPPI POWER CO | Gas marketing services
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
MISSISSIPPI POWER CO | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
MISSISSIPPI POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Operating revenues	6	4	19	14
MISSISSIPPI POWER CO | Other revenue sources
Disaggregation of Revenue [Line Items]
Other	1	(7)	9	(3)
MISSISSIPPI POWER CO | Other adjustments
Disaggregation of Revenue [Line Items]
Other	0	0	0	0
SOUTHERN POWER CO
Disaggregation of Revenue [Line Items]
Operating revenues	332	372	851	980
Revenues	523	574	1,337	1,527
SOUTHERN POWER CO | Total retail electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN POWER CO | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN POWER CO | Transportation
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN POWER CO | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	328	369	840	970
SOUTHERN POWER CO | PPA energy revenues
Disaggregation of Revenue [Line Items]
Operating revenues	165	188	425	508
SOUTHERN POWER CO | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Operating revenues	95	100	231	272
SOUTHERN POWER CO | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Operating revenues	68	81	184	190
SOUTHERN POWER CO | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN POWER CO | Wholesale gas services
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN POWER CO | Gas marketing services
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN POWER CO | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Operating revenues	4	3	11	10
SOUTHERN POWER CO | Other revenue sources
Disaggregation of Revenue [Line Items]
Other	191	202	486	547
SOUTHERN POWER CO | Other adjustments
Disaggregation of Revenue [Line Items]
Other	0	0	0	0
SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Operating revenues	950	913	3,506	4,093
Revenues	477	498	2,362	2,661
SOUTHERN Co GAS | Total retail electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN Co GAS | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN Co GAS | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN Co GAS | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN Co GAS | Other
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN Co GAS | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Operating revenues	474	441	2,058	2,158
SOUTHERN Co GAS | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	170	162	906	992
SOUTHERN Co GAS | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	41	42	229	277
SOUTHERN Co GAS | Transportation
Disaggregation of Revenue [Line Items]
Operating revenues	224	204	723	673
SOUTHERN Co GAS | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	4	3	21	25
SOUTHERN Co GAS | Other
Disaggregation of Revenue [Line Items]
Operating revenues	35	30	179	191
SOUTHERN Co GAS | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN Co GAS | PPA energy revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN Co GAS | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN Co GAS | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN Co GAS | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	476	472	1,448	1,935
SOUTHERN Co GAS | Wholesale gas services
Disaggregation of Revenue [Line Items]
Operating revenues	431	425	1,168	1,590
SOUTHERN Co GAS | Gas marketing services
Disaggregation of Revenue [Line Items]
Operating revenues	38	37	258	313
SOUTHERN Co GAS | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	7	10	22	32
SOUTHERN Co GAS | Other revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0	0
SOUTHERN Co GAS | Other revenue sources
Disaggregation of Revenue [Line Items]
Other	630	727	1,973	2,744
SOUTHERN Co GAS | Other adjustments
Disaggregation of Revenue [Line Items]
Other	$ (1,103)	$ (1,142)	$ (3,117)	$ (4,176)