FAIR VALUE MEASUREMENTS - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Liabilities:
Weather derivative premium	$ 4	$ 8
Collateral already posted, aggregate fair value	99	277
Recurring
Assets:
Cash equivalents	1,395	766
Other investments	30	12
Total	4,189	3,387
Liabilities:
Contingent consideration	19	21
Total	770	1,057
Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	1,393	765
Other investments	9	0
Total	2,650	1,962
Liabilities:
Contingent consideration	0	0
Total	442	648
Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	2	1
Other investments	21	12
Total	1,461	1,380
Liabilities:
Contingent consideration	0	0
Total	302	388
Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Other investments	0	0
Total	22	0
Liabilities:
Contingent consideration	19	21
Total	26	21
Recurring \| Fair Value Measured at Net Asset Value Per Share (NAV)
Assets:
Total	56	45
Alabama Power
Assets:
Derivative asset	2	2
Liabilities:
Derivative liability	22	6
Alabama Power \| Recurring
Assets:
Cash equivalents	693	117
Other investments	21	12
Total	1,739	981
Alabama Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	691	116
Other investments	0	0
Total	1,273	595
Alabama Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	2	1
Other investments	21	12
Total	410	341
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	56	45
Georgia Power
Assets:
Derivative asset	1	0
Liabilities:
Derivative liability	67	17
Georgia Power \| Recurring
Assets:
Total	1,017	879
Liabilities:
Total	70	23
Georgia Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Total	276	224
Liabilities:
Total	0	0
Georgia Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Total	741	655
Liabilities:
Total	70	23
Georgia Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Total	0	0
Liabilities:
Total	0	0
Mississippi Power
Assets:
Derivative asset	0	1
Liabilities:
Derivative liability	26	7
Mississippi Power \| Recurring
Assets:
Cash equivalents	281	255
Total	282	258
Mississippi Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	281	255
Total	281	255
Mississippi Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	0	0
Total	1	3
Mississippi Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Total	0	0
Southern Power
Assets:
Derivative asset	19	76
Liabilities:
Derivative liability	27	28
Southern Power \| Recurring
Assets:
Cash equivalents	113	46
Total	132	125
Liabilities:
Contingent consideration	19	21
Total	46	52
Southern Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	113	46
Total	113	46
Liabilities:
Contingent consideration	0	0
Total	0	0
Southern Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	0	0
Total	19	79
Liabilities:
Contingent consideration	0	0
Total	27	31
Southern Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Total	0	0
Liabilities:
Contingent consideration	19	21
Total	19	21
Southern Company Gas
Assets:
Derivative asset	212	233
Liabilities:
Derivative liability	41	124
Weather derivative premium	4	8
Collateral already posted, aggregate fair value	99	277
Southern Company Gas \| Recurring
Assets:
Cash equivalents	8	40
Total	708	814
Southern Company Gas \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	8	40
Total	397	513
Southern Company Gas \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	0	0
Total	289	301
Southern Company Gas \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Total	22	0
Energy-related derivatives \| Recurring
Assets:
Derivative asset	677	761
Liabilities:
Derivative liability	703	964
Energy-related derivatives \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Derivative asset	388	469
Liabilities:
Derivative liability	442	648
Energy-related derivatives \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Derivative asset	267	292
Liabilities:
Derivative liability	254	316
Energy-related derivatives \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Derivative asset	22	0
Liabilities:
Derivative liability	7	0
Energy-related derivatives \| Alabama Power \| Recurring
Assets:
Derivative asset	4	6
Liabilities:
Derivative liability	24	10
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	4	6
Liabilities:
Derivative liability	24	10
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	4	6
Liabilities:
Derivative liability	53	21
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	4	6
Liabilities:
Derivative liability	53	21
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	1	3
Liabilities:
Derivative liability	27	9
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	1	3
Liabilities:
Derivative liability	27	9
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	3	4
Liabilities:
Derivative liability	3	8
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	3	4
Liabilities:
Derivative liability	3	8
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	665	741
Liabilities:
Derivative liability	596	909
Energy-related derivatives \| Southern Company Gas \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Derivative asset	388	469
Liabilities:
Derivative liability	442	648
Energy-related derivatives \| Southern Company Gas \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Derivative asset	255	272
Liabilities:
Derivative liability	147	261
Energy-related derivatives \| Southern Company Gas \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Derivative asset	22	0
Liabilities:
Derivative liability	7	0
Interest rate derivatives \| Recurring
Assets:
Derivative asset	2
Liabilities:
Derivative liability	24	49
Interest rate derivatives \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Derivative asset	0
Liabilities:
Derivative liability	0	0
Interest rate derivatives \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Derivative asset	2
Liabilities:
Derivative liability	24	49
Interest rate derivatives \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Derivative asset	0
Liabilities:
Derivative liability	0	0
Interest rate derivatives \| Georgia Power \| Recurring
Liabilities:
Derivative liability	17	2
Interest rate derivatives \| Georgia Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Liabilities:
Derivative liability	0	0
Interest rate derivatives \| Georgia Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Liabilities:
Derivative liability	17	2
Interest rate derivatives \| Georgia Power \| Recurring \| Significant Unobservable Inputs (Level 3)
Liabilities:
Derivative liability	0	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	16	75
Liabilities:
Derivative liability	24	23
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	16	75
Liabilities:
Derivative liability	24	23
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	16	75
Liabilities:
Derivative liability	24	23
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	16	75
Liabilities:
Derivative liability	24	23
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	886	708
Domestic equity \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Nuclear decommissioning trusts	751	601
Domestic equity \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Nuclear decommissioning trusts	135	107
Domestic equity \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Nuclear decommissioning trusts	0	0
Domestic equity \| Alabama Power \| Recurring
Assets:
Nuclear decommissioning trusts	611	491
Domestic equity \| Alabama Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Nuclear decommissioning trusts	488	396
Domestic equity \| Alabama Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Nuclear decommissioning trusts	123	95
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	264	206
Domestic equity \| Georgia Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Nuclear decommissioning trusts	263	205
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	11	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	11	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	288	226
Foreign equity \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Nuclear decommissioning trusts	68	53
Foreign equity \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Nuclear decommissioning trusts	220	173
Foreign equity \| Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Nuclear decommissioning trusts	0	0
Foreign equity \| Alabama Power \| Recurring
Assets:
Nuclear decommissioning trusts	132	103
Foreign equity \| Alabama Power \| Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Nuclear decommissioning trusts	68	53
Foreign equity \| Alabama Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Nuclear decommissioning trusts	64	50
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	152	119
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	152	119
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	4	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	4	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	307	261
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	307	261
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	18
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	18
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	286	243
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
U.S. Treasury and government agency securities \| Georgia Power \| Recurring \| Significant Other Observable Inputs (Level 2)
Assets:

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

Dec. 31, 2019

Dec. 31, 2018

Liabilities:

Weather derivative premium

$ 4

$ 8

Collateral already posted, aggregate fair value

277

Recurring

Assets:

Cash equivalents