REVENUE FROM CONTRACTS WITH CUSTOMERS AND LEASE INCOME - Disaggregate Revenue Sources (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Operating revenues	$ 5,018	$ 5,412
Total retail electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	3,219	3,181
Residential
Disaggregation of Revenue [Line Items]
Operating revenues	1,370	1,327
Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	1,146	1,125
Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	680	708
Other
Disaggregation of Revenue [Line Items]
Operating revenues	23	21
Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Operating revenues	999	1,160
Residential
Disaggregation of Revenue [Line Items]
Operating revenues	496	601
Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	130	170
Transportation
Disaggregation of Revenue [Line Items]
Operating revenues	264	256
Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	12	17
Other
Disaggregation of Revenue [Line Items]
Operating revenues	97	116
Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	315	352
PPA energy revenues
Disaggregation of Revenue [Line Items]
Operating revenues	159	190
PPA capacity revenues
Disaggregation of Revenue [Line Items]
Operating revenues	105	107
Non-PPA revenues
Disaggregation of Revenue [Line Items]
Operating revenues	51	55
Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	566	952
Wholesale gas services
Disaggregation of Revenue [Line Items]
Operating revenues	396	721
Gas marketing services
Disaggregation of Revenue [Line Items]
Operating revenues	163	221
Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	7	10
Other revenues
Disaggregation of Revenue [Line Items]
Operating revenues	192	266
Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues	5,291	5,911
Other revenue sources
Disaggregation of Revenue [Line Items]
Operating revenues	868	1,361
Other adjustments
Disaggregation of Revenue [Line Items]
Operating revenues	(1,141)	(1,860)
ALABAMA POWER CO
Disaggregation of Revenue [Line Items]
Operating revenues	1,351	1,408
ALABAMA POWER CO | Total retail electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	1,243	1,271
ALABAMA POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	553	559
ALABAMA POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	364	368
ALABAMA POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	321	338
ALABAMA POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	5	6
ALABAMA POWER CO | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
ALABAMA POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
ALABAMA POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
ALABAMA POWER CO | Transportation
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
ALABAMA POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
ALABAMA POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
ALABAMA POWER CO | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	73	118
ALABAMA POWER CO | PPA energy revenues
Disaggregation of Revenue [Line Items]
Operating revenues	27	31
ALABAMA POWER CO | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Operating revenues	27	27
ALABAMA POWER CO | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Operating revenues	19	60
ALABAMA POWER CO | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
ALABAMA POWER CO | Wholesale gas services
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
ALABAMA POWER CO | Gas marketing services
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
ALABAMA POWER CO | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
ALABAMA POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Operating revenues	37	46
ALABAMA POWER CO | Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues	1,353	1,435
ALABAMA POWER CO | Other revenue sources
Disaggregation of Revenue [Line Items]
Operating revenues	(2)	(27)
ALABAMA POWER CO | Other adjustments
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
GEORGIA POWER CO
Disaggregation of Revenue [Line Items]
Operating revenues	1,825	1,833
GEORGIA POWER CO | Total retail electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	1,777	1,707
GEORGIA POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	760	708
GEORGIA POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	720	692
GEORGIA POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	281	296
GEORGIA POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	16	11
GEORGIA POWER CO | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
GEORGIA POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
GEORGIA POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
GEORGIA POWER CO | Transportation
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
GEORGIA POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
GEORGIA POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
GEORGIA POWER CO | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	23	27
GEORGIA POWER CO | PPA energy revenues
Disaggregation of Revenue [Line Items]
Operating revenues	9	12
GEORGIA POWER CO | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Operating revenues	12	13
GEORGIA POWER CO | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Operating revenues	2	2
GEORGIA POWER CO | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
GEORGIA POWER CO | Wholesale gas services
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
GEORGIA POWER CO | Gas marketing services
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
GEORGIA POWER CO | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
GEORGIA POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Operating revenues	95	92
GEORGIA POWER CO | Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues	1,895	1,826
GEORGIA POWER CO | Other revenue sources
Disaggregation of Revenue [Line Items]
Operating revenues	(70)	7
GEORGIA POWER CO | Other adjustments
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
MISSISSIPPI POWER CO
Disaggregation of Revenue [Line Items]
Operating revenues	277	287
MISSISSIPPI POWER CO | Total retail electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	199	203
MISSISSIPPI POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	57	60
MISSISSIPPI POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	62	65
MISSISSIPPI POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	78	74
MISSISSIPPI POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	2	4
MISSISSIPPI POWER CO | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
MISSISSIPPI POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
MISSISSIPPI POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
MISSISSIPPI POWER CO | Transportation
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
MISSISSIPPI POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
MISSISSIPPI POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
MISSISSIPPI POWER CO | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	72	78
MISSISSIPPI POWER CO | PPA energy revenues
Disaggregation of Revenue [Line Items]
Operating revenues	2	3
MISSISSIPPI POWER CO | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Operating revenues	1	1
MISSISSIPPI POWER CO | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Operating revenues	69	74
MISSISSIPPI POWER CO | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
MISSISSIPPI POWER CO | Wholesale gas services
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
MISSISSIPPI POWER CO | Gas marketing services
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
MISSISSIPPI POWER CO | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
MISSISSIPPI POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Operating revenues	5	5
MISSISSIPPI POWER CO | Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues	276	286
MISSISSIPPI POWER CO | Other revenue sources
Disaggregation of Revenue [Line Items]
Operating revenues	1	1
MISSISSIPPI POWER CO | Other adjustments
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN POWER CO
Disaggregation of Revenue [Line Items]
Operating revenues	375	443
SOUTHERN POWER CO | Total retail electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN POWER CO | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN POWER CO | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN POWER CO | Transportation
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN POWER CO | Other
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN POWER CO | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	249	273
SOUTHERN POWER CO | PPA energy revenues
Disaggregation of Revenue [Line Items]
Operating revenues	125	151
SOUTHERN POWER CO | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Operating revenues	66	81
SOUTHERN POWER CO | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Operating revenues	58	41
SOUTHERN POWER CO | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN POWER CO | Wholesale gas services
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN POWER CO | Gas marketing services
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN POWER CO | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Operating revenues	3	4
SOUTHERN POWER CO | Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues	252	277
SOUTHERN POWER CO | Other revenue sources
Disaggregation of Revenue [Line Items]
Operating revenues	123	166
SOUTHERN POWER CO | Other adjustments
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Operating revenues	1,249	1,474
SOUTHERN Co GAS | Total retail electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN Co GAS | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN Co GAS | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN Co GAS | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN Co GAS | Other
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN Co GAS | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Operating revenues	999	1,160
SOUTHERN Co GAS | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	496	601
SOUTHERN Co GAS | Commercial
Disaggregation of Revenue [Line Items]
Operating revenues	130	170
SOUTHERN Co GAS | Transportation
Disaggregation of Revenue [Line Items]
Operating revenues	264	256
SOUTHERN Co GAS | Industrial
Disaggregation of Revenue [Line Items]
Operating revenues	12	17
SOUTHERN Co GAS | Other
Disaggregation of Revenue [Line Items]
Operating revenues	97	116
SOUTHERN Co GAS | Wholesale electric revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN Co GAS | PPA energy revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN Co GAS | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN Co GAS | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN Co GAS | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	566	952
SOUTHERN Co GAS | Wholesale gas services
Disaggregation of Revenue [Line Items]
Operating revenues	396	721
SOUTHERN Co GAS | Gas marketing services
Disaggregation of Revenue [Line Items]
Operating revenues	163	221
SOUTHERN Co GAS | Other natural gas revenues
Disaggregation of Revenue [Line Items]
Operating revenues	7	10
SOUTHERN Co GAS | Other revenues
Disaggregation of Revenue [Line Items]
Operating revenues	0	0
SOUTHERN Co GAS | Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues	1,565	2,112
SOUTHERN Co GAS | Other revenue sources
Disaggregation of Revenue [Line Items]
Operating revenues	825	1,222
SOUTHERN Co GAS | Other adjustments
Disaggregation of Revenue [Line Items]
Operating revenues	$ (1,141)	$ (1,860)