REVENUE FROM CONTRACTS WITH CUSTOMERS AND LEASE INCOME - Disaggregate Revenue Sources (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Operating revenues	$ 4,765	$ 5,222	$ 10,053	$ 11,131
Revenues	4,620	5,098	9,638	10,510
Total retail electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	3,198	3,546	6,417	6,726
Residential
Disaggregation of Revenue [Line Items]
Operating revenues	1,430	1,491	2,801	2,818
Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	1,103	1,264	2,247	2,390
Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	642	766	1,323	1,474
Other
Disaggregation of Revenue [Line Items]
Operating revenues	23	25	46	44
Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Operating revenues	585	557	1,584	1,717
Residential
Disaggregation of Revenue [Line Items]
Operating revenues	239	229	736	830
Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	58	65	188	235
Transportation
Disaggregation of Revenue [Line Items]
Operating revenues	234	213	499	469
Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	5	5	17	22
Other
Disaggregation of Revenue [Line Items]
Operating revenues	49	45	144	161
Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	325	370	639	723
PPA energy revenues
Disaggregation of Revenue [Line Items]
Operating revenues	167	208	326	398
PPA capacity revenues
Disaggregation of Revenue [Line Items]
Operating revenues	108	109	213	217
Non-PPA revenues
Disaggregation of Revenue [Line Items]
Operating revenues	50	53	100	108
Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	406	511	972	1,463
Wholesale gas services
Disaggregation of Revenue [Line Items]
Operating revenues	341	444	737	1,165
Gas marketing services
Disaggregation of Revenue [Line Items]
Operating revenues	57	55	220	276
Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	8	12	15	22
Other revenues
Disaggregation of Revenue [Line Items]
Operating revenues	251	238	441	502
Other revenue sources
Disaggregation of Revenue [Line Items]
Other	728	1,050	1,599	2,413
Other adjustments
Disaggregation of Revenue [Line Items]
Other	(873)	(1,174)	(2,014)	(3,034)
ALABAMA POWER CO
Disaggregation of Revenue [Line Items]
Operating revenues	1,312	1,490	2,663	2,925
Revenues	1,365	1,513	2,716	2,921
ALABAMA POWER CO | Total retail electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	1,210	1,390	2,454	2,661
ALABAMA POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	549	592	1,103	1,151
ALABAMA POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	353	422	717	790
ALABAMA POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	302	369	623	707
ALABAMA POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	6	7	11	13
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	58	63	131	181
ALABAMA POWER CO | PPA energy revenues
Disaggregation of Revenue [Line Items]
Operating revenues	28	35	55	67
ALABAMA POWER CO | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Operating revenues	25	25	52	52
ALABAMA POWER CO | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Operating revenues	5	3	24	62
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	44	37	78	83
ALABAMA POWER CO | Other revenue sources
Disaggregation of Revenue [Line Items]
Other	53	23	53	(4)
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	1,935	2,069	3,830	3,894
Revenues	1,928	2,117	3,754	3,951
GEORGIA POWER CO | Total retail electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	1,794	1,943	3,572	3,650
GEORGIA POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	817	831	1,577	1,539
GEORGIA POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	689	770	1,409	1,463
GEORGIA POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	273	327	555	623
GEORGIA POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	15	15	31	25
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	22	31	44	58
GEORGIA POWER CO | PPA energy revenues
Disaggregation of Revenue [Line Items]
Operating revenues	7	17	15	28
GEORGIA POWER CO | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Operating revenues	13	13	25	27
GEORGIA POWER CO | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Operating revenues	2	1	4	3
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	119	95	214	186
GEORGIA POWER CO | Other revenue sources
Disaggregation of Revenue [Line Items]
Other	(7)	48	(76)	57
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	276	310	552	596
Revenues	283	313	559	600
MISSISSIPPI POWER CO | Total retail electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	194	213	391	415
MISSISSIPPI POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	64	68	121	128
MISSISSIPPI POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	61	72	121	137
MISSISSIPPI POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	67	70	145	144
MISSISSIPPI POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	2	3	4	6
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	76	93	148	171
MISSISSIPPI POWER CO | PPA energy revenues
Disaggregation of Revenue [Line Items]
Operating revenues	2	3	4	5
MISSISSIPPI POWER CO | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Operating revenues	1	1	2	2
MISSISSIPPI POWER CO | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Operating revenues	73	89	142	164
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	13	10
MISSISSIPPI POWER CO | Other revenue sources
Disaggregation of Revenue [Line Items]
Other	7	3	7	4
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	268	315	519	592
Revenues	439	510	814	953
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	264	312	512	586
SOUTHERN POWER CO | PPA energy revenues
Disaggregation of Revenue [Line Items]
Operating revenues	134	163	259	314
SOUTHERN POWER CO | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Operating revenues	71	81	136	163
SOUTHERN POWER CO | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Operating revenues	59	68	117	109
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	7	6
SOUTHERN POWER CO | Other revenue sources
Disaggregation of Revenue [Line Items]
Other	171	195	295	361
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	991	1,068	2,556	3,180
Revenues	636	689	1,885	2,163
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	585	557	1,584	1,717
SOUTHERN Co GAS | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	239	229	736	830
SOUTHERN Co GAS | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	58	65	188	235
SOUTHERN Co GAS | Transportation
Disaggregation of Revenue [Line Items]
Operating revenues	234	213	499	469
SOUTHERN Co GAS | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	5	5	17	22
SOUTHERN Co GAS | Other
Disaggregation of Revenue [Line Items]
Operating revenues	49	45	144	161
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	406	511	972	1,463
SOUTHERN Co GAS | Wholesale gas services
Disaggregation of Revenue [Line Items]
Operating revenues	341	444	737	1,165
SOUTHERN Co GAS | Gas marketing services
Disaggregation of Revenue [Line Items]
Operating revenues	57	55	220	276
SOUTHERN Co GAS | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	8	12	15	22
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	518	795	1,343	2,017
SOUTHERN Co GAS | Other adjustments
Disaggregation of Revenue [Line Items]
Other	$ (873)	$ (1,174)	$ (2,014)	$ (3,034)