REVENUE FROM CONTRACTS WITH CUSTOMERS - Disaggregate Revenue Sources (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 21,419	$ 23,495	$ 23,031
Retail electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	14,445	15,087
Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	6,164	6,586
Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	5,065	5,255
Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	3,126	3,152
Other
Disaggregation of Revenue [Line Items]
Total operating revenues	90	94
Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	2,989	3,175
Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	1,413	1,525
Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	389	436
Transportation
Disaggregation of Revenue [Line Items]
Total operating revenues	907	944
Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	35	40
Other
Disaggregation of Revenue [Line Items]
Total operating revenues	245	230
Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	1,518	1,711
PPA energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	833	950
PPA capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	453	498
Non-PPA revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	232	263
Other natural gas revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	2,609	3,739
Gas pipeline investments
Disaggregation of Revenue [Line Items]
Total operating revenues	32	32
Wholesale gas services
Disaggregation of Revenue [Line Items]
Total operating revenues	2,095	3,083
Gas marketing services
Disaggregation of Revenue [Line Items]
Total operating revenues	440	571
Other natural gas revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	42	53
Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	1,035	1,529
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total operating revenues	22,596	25,241
Other revenue sources
Disaggregation of Revenue [Line Items]
Total operating revenues	4,266	5,108
Other adjustments
Disaggregation of Revenue [Line Items]
Total operating revenues	(5,443)	(6,854)
Alabama Power
Disaggregation of Revenue [Line Items]
Total operating revenues	6,125	6,032	6,039
Alabama Power | Retail electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	5,671	5,215
Alabama Power | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	2,509	2,285
Alabama Power | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	1,677	1,541
Alabama Power | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	1,460	1,364
Alabama Power | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	25	25
Alabama Power | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Alabama Power | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Alabama Power | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Alabama Power | Transportation
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Alabama Power | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Alabama Power | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Alabama Power | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	328	378
Alabama Power | PPA energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	145	158
Alabama Power | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	102	101
Alabama Power | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	81	119
Alabama Power | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Alabama Power | Gas pipeline investments
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Alabama Power | Wholesale gas services
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Alabama Power | Gas marketing services
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Alabama Power | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Alabama Power | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	153	210
Alabama Power | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total operating revenues	6,152	5,803
Alabama Power | Other revenue sources
Disaggregation of Revenue [Line Items]
Total operating revenues	(27)	229
Alabama Power | Other adjustments
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Georgia Power
Disaggregation of Revenue [Line Items]
Total operating revenues	8,408	8,420	8,310
Georgia Power | Retail electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	7,888	7,697
Georgia Power | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	3,377	3,295
Georgia Power | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	3,097	3,025
Georgia Power | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	1,360	1,321
Georgia Power | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	54	56
Georgia Power | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Georgia Power | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Georgia Power | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Georgia Power | Transportation
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Georgia Power | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Georgia Power | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Georgia Power | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	123	158
Georgia Power | PPA energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	60	81
Georgia Power | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	54	53
Georgia Power | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	9	24
Georgia Power | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Georgia Power | Gas pipeline investments
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Georgia Power | Wholesale gas services
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Georgia Power | Gas marketing services
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Georgia Power | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Georgia Power | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	407	236
Georgia Power | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total operating revenues	8,418	8,091
Georgia Power | Other revenue sources
Disaggregation of Revenue [Line Items]
Total operating revenues	(10)	329
Georgia Power | Other adjustments
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Mississippi Power
Disaggregation of Revenue [Line Items]
Total operating revenues	1,264	1,265	1,187
Mississippi Power | Retail electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	886	902
Mississippi Power | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	278	277
Mississippi Power | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	291	290
Mississippi Power | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	306	326
Mississippi Power | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	11	9
Mississippi Power | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Mississippi Power | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Mississippi Power | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Mississippi Power | Transportation
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Mississippi Power | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Mississippi Power | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Mississippi Power | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	366	350
Mississippi Power | PPA energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	11	15
Mississippi Power | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	3	6
Mississippi Power | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	352	329
Mississippi Power | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Mississippi Power | Gas pipeline investments
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Mississippi Power | Wholesale gas services
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Mississippi Power | Gas marketing services
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Mississippi Power | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Mississippi Power | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	19	22
Mississippi Power | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total operating revenues	1,271	1,274
Mississippi Power | Other revenue sources
Disaggregation of Revenue [Line Items]
Total operating revenues	(7)	(9)
Mississippi Power | Other adjustments
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Power
Disaggregation of Revenue [Line Items]
Total operating revenues	1,938	2,205	2,075
Southern Power | Retail electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Power | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Power | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Power | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Power | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Power | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Power | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Power | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Power | Transportation
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Power | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Power | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Power | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	1,208	1,351
Southern Power | PPA energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	648	727
Southern Power | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	322	394
Southern Power | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	238	230
Southern Power | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Power | Gas pipeline investments
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Power | Wholesale gas services
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Power | Gas marketing services
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Power | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Power | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	12	13
Southern Power | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total operating revenues	1,220	1,364
Southern Power | Other revenue sources
Disaggregation of Revenue [Line Items]
Total operating revenues	718	841
Southern Power | Other adjustments
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Company Gas
Disaggregation of Revenue [Line Items]
Total operating revenues	3,792	3,909	$ 3,920
Southern Company Gas | Retail electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Company Gas | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Company Gas | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Company Gas | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Company Gas | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Company Gas | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	2,989	3,175
Southern Company Gas | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	1,413	1,525
Southern Company Gas | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	389	436
Southern Company Gas | Transportation
Disaggregation of Revenue [Line Items]
Total operating revenues	907	944
Southern Company Gas | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	35	40
Southern Company Gas | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	245	230
Southern Company Gas | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Company Gas | PPA energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Company Gas | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Company Gas | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Company Gas | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	2,609	3,739
Southern Company Gas | Gas pipeline investments
Disaggregation of Revenue [Line Items]
Total operating revenues	32	32
Southern Company Gas | Wholesale gas services
Disaggregation of Revenue [Line Items]
Total operating revenues	2,095	3,083
Southern Company Gas | Gas marketing services
Disaggregation of Revenue [Line Items]
Total operating revenues	440	571
Southern Company Gas | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	42	53
Southern Company Gas | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Southern Company Gas | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total operating revenues	5,598	6,914
Southern Company Gas | Other revenue sources
Disaggregation of Revenue [Line Items]
Total operating revenues	3,637	3,849
Southern Company Gas | Other adjustments
Disaggregation of Revenue [Line Items]
Total operating revenues	$ (5,443)	$ (6,854)