REVENUE FROM CONTRACTS WITH CUSTOMERS - Disaggregate Revenue Sources (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 21,097	$ 22,596	$ 25,241
Total operating revenues	20,375	21,419	23,495
Retail electric revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	13,677	14,445	15,087
Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	6,113	6,164	6,586
Commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4,699	5,065	5,255
Industrial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,775	3,126	3,152
Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	90	90	94
Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,888	2,989	3,175
Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,338	1,413	1,525
Commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	340	389	436
Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	971	907	944
Industrial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	30	35	40
Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	209	245	230
Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,399	1,518	1,711
PPA energy revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	735	833	950
PPA capacity revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	454	453	498
Non-PPA revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	210	232	263
Other natural gas revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,151	2,609	3,739
Gas pipeline investments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		32	32
Wholesale gas services
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,727	2,095	3,083
Gas marketing services
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	391	440	571
Other natural gas revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	33	42	53
Other revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	982	1,035	1,529
Other revenue sources
Disaggregation of Revenue [Line Items]
Other	3,764	4,266	5,108
Other adjustments
Disaggregation of Revenue [Line Items]
Other	(4,486)	(5,443)	(6,854)
Alabama Power
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	5,646	6,152	5,803
Total operating revenues	5,830	6,125	6,032
Alabama Power | Retail electric revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	5,203	5,671	5,215
Alabama Power | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,377	2,509	2,285
Alabama Power | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,512	1,677	1,541
Alabama Power | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,293	1,460	1,364
Alabama Power | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	21	25	25
Alabama Power | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Alabama Power | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Alabama Power | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Alabama Power | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Alabama Power | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Alabama Power | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Alabama Power | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	284	328	378
Alabama Power | PPA energy revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	133	145	158
Alabama Power | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	108	102	101
Alabama Power | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	43	81	119
Alabama Power | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Alabama Power | Gas pipeline investments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		0	0
Alabama Power | Wholesale gas services
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Alabama Power | Gas marketing services
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Alabama Power | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Alabama Power | Other revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	159	153	210
Alabama Power | Other revenue sources
Disaggregation of Revenue [Line Items]
Other	184	(27)	229
Alabama Power | Other adjustments
Disaggregation of Revenue [Line Items]
Other	0	0	0
Georgia Power
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	8,215	8,418	8,091
Total operating revenues	8,309	8,408	8,420
Georgia Power | Retail electric revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	7,666	7,888	7,697
Georgia Power | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3,476	3,377	3,295
Georgia Power | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,933	3,097	3,025
Georgia Power | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,197	1,360	1,321
Georgia Power | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	60	54	56
Georgia Power | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Georgia Power | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Georgia Power | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Georgia Power | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Georgia Power | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Georgia Power | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Georgia Power | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	102	123	158
Georgia Power | PPA energy revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	42	60	81
Georgia Power | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	50	54	53
Georgia Power | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	10	9	24
Georgia Power | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Georgia Power | Gas pipeline investments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		0	0
Georgia Power | Wholesale gas services
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Georgia Power | Gas marketing services
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Georgia Power | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Georgia Power | Other revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	447	407	236
Georgia Power | Other revenue sources
Disaggregation of Revenue [Line Items]
Other	94	(10)	329
Georgia Power | Other adjustments
Disaggregation of Revenue [Line Items]
Other	0	0	0
Mississippi Power
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,157	1,271	1,274
Total operating revenues	1,172	1,264	1,265
Mississippi Power | Retail electric revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	808	886	902
Mississippi Power | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	260	278	277
Mississippi Power | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	254	291	290
Mississippi Power | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	285	306	326
Mississippi Power | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	9	11	9
Mississippi Power | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Mississippi Power | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Mississippi Power | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Mississippi Power | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Mississippi Power | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Mississippi Power | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Mississippi Power | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	323	366	350
Mississippi Power | PPA energy revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	9	11	15
Mississippi Power | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3	3	6
Mississippi Power | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	311	352	329
Mississippi Power | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Mississippi Power | Gas pipeline investments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		0	0
Mississippi Power | Wholesale gas services
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Mississippi Power | Gas marketing services
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Mississippi Power | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Mississippi Power | Other revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	26	19	22
Mississippi Power | Other revenue sources
Disaggregation of Revenue [Line Items]
Other	15	(7)	(9)
Mississippi Power | Other adjustments
Disaggregation of Revenue [Line Items]
Other	0	0	0
Southern Power
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,119	1,220	1,364
Total operating revenues	1,733	1,938	2,205
Southern Power | Retail electric revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Power | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Power | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Power | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Power | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Power | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Power | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Power | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Power | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Power | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Power | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Power | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,105	1,208	1,351
Southern Power | PPA energy revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	570	648	727
Southern Power | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	296	322	394
Southern Power | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	239	238	230
Southern Power | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Power | Gas pipeline investments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		0	0
Southern Power | Wholesale gas services
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Power | Gas marketing services
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Power | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Power | Other revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	14	12	13
Southern Power | Other revenue sources
Disaggregation of Revenue [Line Items]
Other	614	718	841
Southern Power | Other adjustments
Disaggregation of Revenue [Line Items]
Other	0	0	0
Southern Company Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	5,039	5,598	6,914
Total operating revenues	3,434	3,792	3,909
Southern Company Gas | Retail electric revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Company Gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Company Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Company Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Company Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Company Gas | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,888	2,989	3,175
Southern Company Gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,338	1,413	1,525
Southern Company Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	340	389	436
Southern Company Gas | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	971	907	944
Southern Company Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	30	35	40
Southern Company Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	209	245	230
Southern Company Gas | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Company Gas | PPA energy revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Company Gas | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Company Gas | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Company Gas | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,151	2,609	3,739
Southern Company Gas | Gas pipeline investments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		32	32
Southern Company Gas | Wholesale gas services
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,727	2,095	3,083
Southern Company Gas | Gas marketing services
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	391	440	571
Southern Company Gas | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	33	42	53
Southern Company Gas | Other revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Southern Company Gas | Other revenue sources
Disaggregation of Revenue [Line Items]
Other	2,881	3,637	3,849
Southern Company Gas | Other adjustments
Disaggregation of Revenue [Line Items]
Other	$ (4,486)	$ (5,443)	$ (6,854)