INCOME TAXES - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred tax liabilities —
Deferred income tax liabilities	$ 16,781	$ 16,261
Deferred tax assets —
Deferred income tax assets	8,874	8,680
Valuation allowance	(136)	(137)
Net deferred income tax assets	8,738	8,543
Net deferred income taxes liabilities	8,043	7,718
Accumulated deferred income taxes – assets	(132)	(170)
Accumulated deferred income taxes – liabilities	8,175	7,888
Accelerated depreciation
Deferred tax liabilities —
Deferred income tax liabilities	8,950	8,711
Property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	1,999	1,843
Federal effect of net state deferred tax assets/liabilities
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Deferred tax assets —
Deferred income tax assets	284	277
Leveraged lease basis differences
Deferred tax liabilities —
Deferred income tax liabilities	142	236
Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	739	704
Deferred tax assets —
Deferred income tax assets	1,511	1,385
Premium on reacquired debt
Deferred tax liabilities —
Deferred income tax liabilities	78	83
Storm damage reserves
Deferred tax liabilities —
Deferred income tax liabilities	80	109
Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	1,313	1,174
Remaining book value of retired assets
Deferred tax liabilities —
Deferred income tax liabilities	270	341
AROs
Deferred tax liabilities —
Deferred income tax liabilities	1,969	1,723
AROs
Deferred tax liabilities —
Deferred income tax liabilities	804	814
Other
Deferred tax liabilities —
Deferred income tax liabilities	437	523
Other property basis differences
Deferred tax assets —
Deferred income tax assets	223	230
ITC and PTC carryforward
Deferred tax assets —
Deferred income tax assets	1,853	2,098
Long-term debt fair value adjustment
Deferred tax assets —
Deferred income tax assets	86	97
Other partnership basis difference
Deferred tax assets —
Deferred income tax assets	166	169
Other comprehensive losses
Deferred tax assets —
Deferred income tax assets	128	112
AROs
Deferred tax assets —
Deferred income tax assets	2,773	2,537
Estimated loss on plants under construction
Deferred tax assets —
Deferred income tax assets	369	283
Other deferred state tax attributes
Deferred tax assets —
Deferred income tax assets	357	402
Regulatory liability associated with the Tax Reform Legislation (not subject to normalization)
Deferred tax assets —
Deferred income tax assets	338	401
Other
Deferred tax assets —
Deferred income tax assets	660	689
Alabama Power
Deferred tax liabilities —
Deferred income tax liabilities	5,416	5,161
Deferred tax assets —
Deferred income tax assets	2,143	1,901
Valuation allowance	0	0
Net deferred income tax assets	2,143	1,901
Net deferred income taxes liabilities	3,273	3,260
Accumulated deferred income taxes – assets	0	0
Accumulated deferred income taxes – liabilities	3,273	3,260
Alabama Power | Accelerated depreciation
Deferred tax liabilities —
Deferred income tax liabilities	2,453	2,402
Alabama Power | Property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	1,010	912
Alabama Power | Federal effect of net state deferred tax assets/liabilities
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Deferred tax assets —
Deferred income tax assets	151	162
Alabama Power | Leveraged lease basis differences
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Alabama Power | Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	250	242
Deferred tax assets —
Deferred income tax assets	369	334
Alabama Power | Premium on reacquired debt
Deferred tax liabilities —
Deferred income tax liabilities	12	13
Alabama Power | Storm damage reserves
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Alabama Power | Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	348	311
Alabama Power | Remaining book value of retired assets
Deferred tax liabilities —
Deferred income tax liabilities	123	174
Alabama Power | AROs
Deferred tax liabilities —
Deferred income tax liabilities	764	613
Alabama Power | AROs
Deferred tax liabilities —
Deferred income tax liabilities	328	360
Alabama Power | Other
Deferred tax liabilities —
Deferred income tax liabilities	128	134
Alabama Power | Other property basis differences
Deferred tax assets —
Deferred income tax assets	0	0
Alabama Power | ITC and PTC carryforward
Deferred tax assets —
Deferred income tax assets	12	11
Alabama Power | Long-term debt fair value adjustment
Deferred tax assets —
Deferred income tax assets	0	0
Alabama Power | Other partnership basis difference
Deferred tax assets —
Deferred income tax assets	0	0
Alabama Power | Other comprehensive losses
Deferred tax assets —
Deferred income tax assets	7	8
Alabama Power | AROs
Deferred tax assets —
Deferred income tax assets	1,092	973
Alabama Power | Estimated loss on plants under construction
Deferred tax assets —
Deferred income tax assets	0	0
Alabama Power | Other deferred state tax attributes
Deferred tax assets —
Deferred income tax assets	0	0
Alabama Power | Regulatory liability associated with the Tax Reform Legislation (not subject to normalization)
Deferred tax assets —
Deferred income tax assets	243	240
Alabama Power | Other
Deferred tax assets —
Deferred income tax assets	143	173
Georgia Power
Deferred tax liabilities —
Deferred income tax liabilities	6,680	6,345
Deferred tax assets —
Deferred income tax assets	3,443	3,123
Valuation allowance	(35)	(35)
Net deferred income tax assets	3,408	3,088
Net deferred income taxes liabilities	3,272	3,257
Accumulated deferred income taxes – assets	0	0
Accumulated deferred income taxes – liabilities	3,272	3,257
Georgia Power | Accelerated depreciation
Deferred tax liabilities —
Deferred income tax liabilities	3,228	3,058
Georgia Power | Property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	689	643
Georgia Power | Federal effect of net state deferred tax assets/liabilities
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Deferred tax assets —
Deferred income tax assets	59	63
Georgia Power | Leveraged lease basis differences
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Georgia Power | Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	362	351
Deferred tax assets —
Deferred income tax assets	522	488
Georgia Power | Premium on reacquired debt
Deferred tax liabilities —
Deferred income tax liabilities	66	70
Georgia Power | Storm damage reserves
Deferred tax liabilities —
Deferred income tax liabilities	80	109
Georgia Power | Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	438	403
Georgia Power | Remaining book value of retired assets
Deferred tax liabilities —
Deferred income tax liabilities	141	159
Georgia Power | AROs
Deferred tax liabilities —
Deferred income tax liabilities	1,165	1,066
Georgia Power | AROs
Deferred tax liabilities —
Deferred income tax liabilities	429	405
Georgia Power | Other
Deferred tax liabilities —
Deferred income tax liabilities	82	81
Georgia Power | Other property basis differences
Deferred tax assets —
Deferred income tax assets	72	65
Georgia Power | ITC and PTC carryforward
Deferred tax assets —
Deferred income tax assets	539	435
Georgia Power | Long-term debt fair value adjustment
Deferred tax assets —
Deferred income tax assets	0	0
Georgia Power | Other partnership basis difference
Deferred tax assets —
Deferred income tax assets	0	0
Georgia Power | Other comprehensive losses
Deferred tax assets —
Deferred income tax assets	17	18
Georgia Power | AROs
Deferred tax assets —
Deferred income tax assets	1,594	1,471
Georgia Power | Estimated loss on plants under construction
Deferred tax assets —
Deferred income tax assets	369	283
Georgia Power | Other deferred state tax attributes
Deferred tax assets —
Deferred income tax assets	9	13
Georgia Power | Regulatory liability associated with the Tax Reform Legislation (not subject to normalization)
Deferred tax assets —
Deferred income tax assets	76	133
Georgia Power | Other
Deferred tax assets —
Deferred income tax assets	186	154
Mississippi Power
Deferred tax liabilities —
Deferred income tax liabilities	705	695
Deferred tax assets —
Deferred income tax assets	428	451
Valuation allowance	(41)	(41)
Net deferred income tax assets	387	410
Net deferred income taxes liabilities	318	285
Accumulated deferred income taxes – assets	(129)	(139)
Accumulated deferred income taxes – liabilities	447	424
Mississippi Power | Accelerated depreciation
Deferred tax liabilities —
Deferred income tax liabilities	319	315
Mississippi Power | Property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	148	143
Mississippi Power | Federal effect of net state deferred tax assets/liabilities
Deferred tax liabilities —
Deferred income tax liabilities	25	24
Deferred tax assets —
Deferred income tax assets	0	0
Mississippi Power | Leveraged lease basis differences
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Mississippi Power | Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	39	38
Deferred tax assets —
Deferred income tax assets	80	72
Mississippi Power | Premium on reacquired debt
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Mississippi Power | Storm damage reserves
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Mississippi Power | Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	62	55
Mississippi Power | Remaining book value of retired assets
Deferred tax liabilities —
Deferred income tax liabilities	6	8
Mississippi Power | AROs
Deferred tax liabilities —
Deferred income tax liabilities	40	44
Mississippi Power | AROs
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Mississippi Power | Other
Deferred tax liabilities —
Deferred income tax liabilities	66	68
Mississippi Power | Other property basis differences
Deferred tax assets —
Deferred income tax assets	0	0
Mississippi Power | ITC and PTC carryforward
Deferred tax assets —
Deferred income tax assets	0	0
Mississippi Power | Long-term debt fair value adjustment
Deferred tax assets —
Deferred income tax assets	0	0
Mississippi Power | Other partnership basis difference
Deferred tax assets —
Deferred income tax assets	0	0
Mississippi Power | Other comprehensive losses
Deferred tax assets —
Deferred income tax assets	0	0
Mississippi Power | AROs
Deferred tax assets —
Deferred income tax assets	40	44
Mississippi Power | Estimated loss on plants under construction
Deferred tax assets —
Deferred income tax assets	0	0
Mississippi Power | Other deferred state tax attributes
Deferred tax assets —
Deferred income tax assets	250	251
Mississippi Power | Regulatory liability associated with the Tax Reform Legislation (not subject to normalization)
Deferred tax assets —
Deferred income tax assets	19	28
Mississippi Power | Other
Deferred tax assets —
Deferred income tax assets	39	56
Southern Power
Deferred tax liabilities —
Deferred income tax liabilities	1,413	1,445
Deferred tax assets —
Deferred income tax assets	1,587	1,917
Valuation allowance	(35)	(36)
Net deferred income tax assets	1,552	1,881
Net deferred income taxes assets	(139)	(436)
Accumulated deferred income taxes – assets	(262)	(551)
Accumulated deferred income taxes – liabilities	123	115
Southern Power | Accelerated depreciation
Deferred tax liabilities —
Deferred income tax liabilities	1,389	1,422
Southern Power | Property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Southern Power | Federal effect of net state deferred tax assets/liabilities
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Deferred tax assets —
Deferred income tax assets	26	24
Southern Power | Leveraged lease basis differences
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Southern Power | Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	12	12
Deferred tax assets —
Deferred income tax assets	6	5
Southern Power | Premium on reacquired debt
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Southern Power | Storm damage reserves
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Southern Power | Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Southern Power | Remaining book value of retired assets
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Southern Power | AROs
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Southern Power | AROs
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Southern Power | Other
Deferred tax liabilities —
Deferred income tax liabilities	12	11
Southern Power | Other property basis differences
Deferred tax assets —
Deferred income tax assets	134	146
Southern Power | ITC and PTC carryforward
Deferred tax assets —
Deferred income tax assets	1,110	1,445
Southern Power | Long-term debt fair value adjustment
Deferred tax assets —
Deferred income tax assets	0	0
Southern Power | Other partnership basis difference
Deferred tax assets —
Deferred income tax assets	166	169
Southern Power | Other comprehensive losses
Deferred tax assets —
Deferred income tax assets	25	10
Southern Power | AROs
Deferred tax assets —
Deferred income tax assets	0	0
Southern Power | Estimated loss on plants under construction
Deferred tax assets —
Deferred income tax assets	0	0
Southern Power | Other deferred state tax attributes
Deferred tax assets —
Deferred income tax assets	68	72
Southern Power | Regulatory liability associated with the Tax Reform Legislation (not subject to normalization)
Deferred tax assets —
Deferred income tax assets	0	0
Southern Power | Other
Deferred tax assets —
Deferred income tax assets	52	46
Southern Company Gas
Deferred tax liabilities —
Deferred income tax liabilities	1,693	1,676
Deferred tax assets —
Deferred income tax assets	432	462
Valuation allowance	(4)	(5)
Net deferred income tax assets	428	457
Net deferred income taxes liabilities	1,265	1,219
Accumulated deferred income taxes – assets	0	0
Accumulated deferred income taxes – liabilities	1,265	1,219
Southern Company Gas | Accelerated depreciation
Deferred tax liabilities —
Deferred income tax liabilities	1,349	1,288
Southern Company Gas | Property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	135	133
Southern Company Gas | Federal effect of net state deferred tax assets/liabilities
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Deferred tax assets —
Deferred income tax assets	70	56
Southern Company Gas | Leveraged lease basis differences
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Southern Company Gas | Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	26	12
Deferred tax assets —
Deferred income tax assets	100	111
Southern Company Gas | Premium on reacquired debt
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Southern Company Gas | Storm damage reserves
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Southern Company Gas | Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	45	45
Southern Company Gas | Remaining book value of retired assets
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Southern Company Gas | AROs
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Southern Company Gas | AROs
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Southern Company Gas | Other
Deferred tax liabilities —
Deferred income tax liabilities	138	198
Southern Company Gas | Other property basis differences
Deferred tax assets —
Deferred income tax assets	0	0
Southern Company Gas | ITC and PTC carryforward
Deferred tax assets —
Deferred income tax assets	0	0
Southern Company Gas | Long-term debt fair value adjustment
Deferred tax assets —
Deferred income tax assets	86	97
Southern Company Gas | Other partnership basis difference
Deferred tax assets —
Deferred income tax assets	0	0
Southern Company Gas | Other comprehensive losses
Deferred tax assets —
Deferred income tax assets	0	0
Southern Company Gas | AROs
Deferred tax assets —
Deferred income tax assets	0	0
Southern Company Gas | Estimated loss on plants under construction
Deferred tax assets —
Deferred income tax assets	0	0
Southern Company Gas | Other deferred state tax attributes
Deferred tax assets —
Deferred income tax assets	10	8
Southern Company Gas | Regulatory liability associated with the Tax Reform Legislation (not subject to normalization)
Deferred tax assets —
Deferred income tax assets	0	0
Southern Company Gas | Other
Deferred tax assets —
Deferred income tax assets	$ 166	$ 190