FAIR VALUE MEASUREMENTS - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets:
Derivative asset	$ 281	$ 233
Liabilities:
Derivative liability	64	189
Weather derivative premium	6	4
Collateral already posted, aggregate fair value	28	99
Recurring
Assets:
Cash equivalents	584	1,395
Other investments	33	30
Total	3,759	4,189
Liabilities:
Contingent consideration	17	19
Total	637	770
Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	575	1,393
Other investments	9	9
Total	1,974	2,650
Liabilities:
Contingent consideration	0	0
Total	389	442
Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	9	2
Other investments	24	21
Total	1,677	1,461
Liabilities:
Contingent consideration	0	0
Total	227	302
Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Other investments	0	0
Total	32	22
Liabilities:
Contingent consideration	17	19
Total	21	26
Recurring \| Fair Value Measured at Net Asset Value Per Share (NAV)
Assets:
Total	76	56
Alabama Power
Assets:
Derivative asset	5	2
Liabilities:
Derivative liability	0	22
Alabama Power \| Recurring
Assets:
Cash equivalents	320	693
Other investments	24	21
Total	1,512	1,739
Alabama Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	311	691
Other investments	0	0
Total	959	1,273
Alabama Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	9	2
Other investments	24	21
Total	477	410
Alabama Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Other investments	0	0
Total	0	0
Alabama Power \| Recurring \| Fair Value Measured at Net Asset Value Per Share (NAV)
Assets:
Total	76	56
Georgia Power
Assets:
Derivative asset	3	1
Liabilities:
Derivative liability	1	67
Georgia Power \| Recurring
Assets:
Total	1,160	1,017
Liabilities:
Total		70
Georgia Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Total	340	276
Liabilities:
Total		0
Georgia Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Total	820	741
Liabilities:
Total		70
Georgia Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Total	0	0
Liabilities:
Total		0
Mississippi Power
Assets:
Derivative asset	2	0
Liabilities:
Derivative liability	2	26
Mississippi Power \| Recurring
Assets:
Cash equivalents	21	281
Total	30	282
Mississippi Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	21	281
Total	21	281
Mississippi Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	0	0
Total	9	1
Mississippi Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Total	0	0
Southern Power
Assets:
Derivative asset	89	19
Liabilities:
Derivative liability	26	27
Southern Power \| Recurring
Assets:
Cash equivalents		113
Total	89	132
Liabilities:
Contingent consideration	17	19
Total	43	46
Southern Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents		113
Total	0	113
Liabilities:
Contingent consideration	0	0
Total	0	0
Southern Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents		0
Total	89	19
Liabilities:
Contingent consideration	0	0
Total	26	27
Southern Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents		0
Total	0	0
Liabilities:
Contingent consideration	17	19
Total	17	19
Southern Company Gas
Assets:
Derivative asset	162	212
Liabilities:
Derivative liability	35	41
Collateral already posted, aggregate fair value	28	99
Southern Company Gas \| Recurring
Assets:
Cash equivalents		8
Total	696	708
Southern Company Gas \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents		8
Total	402	397
Southern Company Gas \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents		0
Total	262	289
Southern Company Gas \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents		0
Total	32	22
Energy-related derivatives \| Recurring
Assets:
Derivative asset	704	677
Liabilities:
Derivative liability	597	703
Energy-related derivatives \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Derivative asset	401	388
Liabilities:
Derivative liability	389	442
Energy-related derivatives \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Derivative asset	271	267
Liabilities:
Derivative liability	204	254
Energy-related derivatives \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Derivative asset	32	22
Liabilities:
Derivative liability	4	7
Energy-related derivatives \| Alabama Power \| Recurring
Assets:
Derivative asset	12	4
Liabilities:
Derivative liability	7	24
Energy-related derivatives \| Alabama Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Derivative asset	0	0
Liabilities:
Derivative liability	0	0
Energy-related derivatives \| Alabama Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Derivative asset	12	4
Liabilities:
Derivative liability	7	24
Energy-related derivatives \| Alabama Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Derivative asset	0	0
Liabilities:
Derivative liability	0	0
Energy-related derivatives \| Georgia Power \| Recurring
Assets:
Derivative asset	15	4
Liabilities:
Derivative liability	13	53
Energy-related derivatives \| Georgia Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Derivative asset	0	0
Liabilities:
Derivative liability	0	0
Energy-related derivatives \| Georgia Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Derivative asset	15	4
Liabilities:
Derivative liability	13	53
Energy-related derivatives \| Georgia Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Derivative asset	0	0
Liabilities:
Derivative liability	0	0
Energy-related derivatives \| Mississippi Power \| Recurring
Assets:
Derivative asset	9	1
Liabilities:
Derivative liability	9	27
Energy-related derivatives \| Mississippi Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Derivative asset	0	0
Liabilities:
Derivative liability	0	0
Energy-related derivatives \| Mississippi Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Derivative asset	9	1
Liabilities:
Derivative liability	9	27
Energy-related derivatives \| Mississippi Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Derivative asset	0	0
Liabilities:
Derivative liability	0	0
Energy-related derivatives \| Southern Power \| Recurring
Assets:
Derivative asset	2	3
Liabilities:
Derivative liability	3	3
Energy-related derivatives \| Southern Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Derivative asset	0	0
Liabilities:
Derivative liability	0	0
Energy-related derivatives \| Southern Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Derivative asset	2	3
Liabilities:
Derivative liability	3	3
Energy-related derivatives \| Southern Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Derivative asset	0	0
Liabilities:
Derivative liability	0	0
Energy-related derivatives \| Southern Company Gas \| Recurring
Assets:
Derivative asset	666	665
Liabilities:
Derivative liability	565	596
Energy-related derivatives \| Southern Company Gas \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Derivative asset	401	388
Liabilities:
Derivative liability	389	442
Energy-related derivatives \| Southern Company Gas \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Derivative asset	233	255
Liabilities:
Derivative liability	172	147
Energy-related derivatives \| Southern Company Gas \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Derivative asset	32	22
Liabilities:
Derivative liability	4	7
Interest rate derivatives \| Recurring
Assets:
Derivative asset	20	2
Liabilities:
Derivative liability		24
Interest rate derivatives \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Derivative asset	0	0
Liabilities:
Derivative liability		0
Interest rate derivatives \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Derivative asset	20	2
Liabilities:
Derivative liability		24
Interest rate derivatives \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Derivative asset	0	0
Liabilities:
Derivative liability		0
Interest rate derivatives \| Georgia Power \| Recurring
Liabilities:
Derivative liability		17
Interest rate derivatives \| Georgia Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Liabilities:
Derivative liability		0
Interest rate derivatives \| Georgia Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Liabilities:
Derivative liability		17
Interest rate derivatives \| Georgia Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Liabilities:
Derivative liability		0
Interest rate derivatives \| Southern Company Gas \| Recurring
Assets:
Derivative asset		2
Interest rate derivatives \| Southern Company Gas \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Derivative asset		0
Interest rate derivatives \| Southern Company Gas \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Derivative asset		2
Interest rate derivatives \| Southern Company Gas \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Derivative asset		0
Foreign currency derivatives \| Recurring
Assets:
Derivative asset	87	16
Liabilities:
Derivative liability	23	24
Foreign currency derivatives \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Derivative asset	0	0
Liabilities:
Derivative liability	0	0
Foreign currency derivatives \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Derivative asset	87	16
Liabilities:
Derivative liability	23	24
Foreign currency derivatives \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Derivative asset	0	0
Liabilities:
Derivative liability	0	0
Foreign currency derivatives \| Southern Power \| Recurring
Assets:
Derivative asset	87	16
Liabilities:
Derivative liability	23	24
Foreign currency derivatives \| Southern Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Derivative asset	0	0
Liabilities:
Derivative liability	0	0
Foreign currency derivatives \| Southern Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Derivative asset	87	16
Liabilities:
Derivative liability	23	24
Foreign currency derivatives \| Southern Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Derivative asset	0	0
Liabilities:
Derivative liability	0	0
Domestic equity \| Recurring
Assets:
Nuclear decommissioning trusts	1,013	886
Domestic equity \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Nuclear decommissioning trusts	862	751
Domestic equity \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Nuclear decommissioning trusts	151	135
Domestic equity \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Nuclear decommissioning trusts	0	0
Domestic equity \| Alabama Power \| Recurring
Assets:
Nuclear decommissioning trusts	684	611
Domestic equity \| Alabama Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Nuclear decommissioning trusts	543	488
Domestic equity \| Alabama Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Nuclear decommissioning trusts	141	123
Domestic equity \| Alabama Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Nuclear decommissioning trusts	0	0
Domestic equity \| Georgia Power \| Recurring
Assets:
Nuclear decommissioning trusts	320	264
Domestic equity \| Georgia Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Nuclear decommissioning trusts	319	263
Domestic equity \| Georgia Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Nuclear decommissioning trusts	1	1
Domestic equity \| Georgia Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Nuclear decommissioning trusts	0	0
Domestic equity \| Southern Company Gas \| Recurring
Assets:
Non-qualified deferred compensation trusts	9	11
Domestic equity \| Southern Company Gas \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Non-qualified deferred compensation trusts	0	0
Domestic equity \| Southern Company Gas \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Non-qualified deferred compensation trusts	9	11
Domestic equity \| Southern Company Gas \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Non-qualified deferred compensation trusts	0	0
Foreign equity \| Recurring
Assets:
Nuclear decommissioning trusts	338	288
Foreign equity \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Nuclear decommissioning trusts	85	68
Foreign equity \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Nuclear decommissioning trusts	253	220
Foreign equity \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Nuclear decommissioning trusts	0	0
Foreign equity \| Alabama Power \| Recurring
Assets:
Nuclear decommissioning trusts	158	132
Foreign equity \| Alabama Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Nuclear decommissioning trusts	85	68
Foreign equity \| Alabama Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Nuclear decommissioning trusts	73	64
Foreign equity \| Alabama Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Nuclear decommissioning trusts	0	0
Foreign equity \| Georgia Power \| Recurring
Assets:
Nuclear decommissioning trusts	177	152
Foreign equity \| Georgia Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Nuclear decommissioning trusts	0	0
Foreign equity \| Georgia Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Nuclear decommissioning trusts	177	152
Foreign equity \| Georgia Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Nuclear decommissioning trusts	0	0
Foreign equity \| Southern Company Gas \| Recurring
Assets:
Non-qualified deferred compensation trusts	3	4
Foreign equity \| Southern Company Gas \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Non-qualified deferred compensation trusts	0	0
Foreign equity \| Southern Company Gas \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Non-qualified deferred compensation trusts	3	4
Foreign equity \| Southern Company Gas \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Non-qualified deferred compensation trusts	0	0
U.S. Treasury and government agency securities \| Recurring
Assets:
Nuclear decommissioning trusts	284	307
U.S. Treasury and government agency securities \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Nuclear decommissioning trusts	0	0
U.S. Treasury and government agency securities \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Nuclear decommissioning trusts	284	307
U.S. Treasury and government agency securities \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Nuclear decommissioning trusts	0	0
U.S. Treasury and government agency securities \| Alabama Power \| Recurring
Assets:
Nuclear decommissioning trusts	21	21
U.S. Treasury and government agency securities \| Alabama Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Nuclear decommissioning trusts	0	0
U.S. Treasury and government agency securities \| Alabama Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Nuclear decommissioning trusts	21	21
U.S. Treasury and government agency securities \| Alabama Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Nuclear decommissioning trusts	0	0
U.S. Treasury and government agency securities \| Georgia Power \| Recurring
Assets:
Nuclear decommissioning trusts	263	286
U.S. Treasury and government agency securities \| Georgia Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Nuclear decommissioning trusts	0	0

FAIR VALUE MEASUREMENTS - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Millions

Dec. 31, 2020

Dec. 31, 2019

Assets:

Derivative asset

$ 281

$ 233

Liabilities:

Derivative liability

189

Weather derivative premium

Collateral already posted, aggregate fair value