INCOME TAXES - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Deferred tax liabilities —
Deferred income tax liabilities	$ 16,261	$ 15,207
Deferred tax assets —
Deferred income tax assets	8,680	9,048
Valuation allowance	(137)	(123)
Net deferred income tax assets	8,543	8,925
Net deferred income taxes liabilities	7,718	6,282
Accumulated deferred income taxes – assets	(170)	(276)
Accumulated deferred income taxes – liabilities	7,888	6,558
Accelerated depreciation
Deferred tax liabilities —
Deferred income tax liabilities	8,711	8,461
Property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	1,843	1,807
Federal effect of net state deferred tax liabilities
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Deferred tax assets —
Deferred income tax assets	277	260
Leveraged lease basis differences
Deferred tax liabilities —
Deferred income tax liabilities	236	253
Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	704	477
Deferred tax assets —
Deferred income tax assets	1,385	1,273
Premium on reacquired debt
Deferred tax liabilities —
Deferred income tax liabilities	83	88
Storm damage reserves
Deferred tax liabilities —
Deferred income tax liabilities	109	111
Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	1,174	975
Regulatory assets associated with remaining book value of retired assets
Deferred tax liabilities —
Deferred income tax liabilities	341	56
AROs
Deferred tax liabilities —
Deferred income tax liabilities	1,723	1,232
AROs
Deferred tax liabilities —
Deferred income tax liabilities	814	1,210
Other
Deferred tax liabilities —
Deferred income tax liabilities	523	537
Other property basis differences
Deferred tax assets —
Deferred income tax assets	230	251
ITC and PTC carryforward
Deferred tax assets —
Deferred income tax assets	2,098	2,730
Alternative minimum tax carryforward
Deferred tax assets —
Deferred income tax assets		62
Other partnership basis difference
Deferred tax assets —
Deferred income tax assets	169	162
Other comprehensive losses
Deferred tax assets —
Deferred income tax assets	112	82
AROs
Deferred tax assets —
Deferred income tax assets	2,537	2,442
Estimated loss on plants under construction
Deferred tax assets —
Deferred income tax assets	283	346
Other deferred state tax attributes
Deferred tax assets —
Deferred income tax assets	402	415
Regulatory liability associated with the Tax Reform Legislation (not subject to normalization)
Deferred tax assets —
Deferred income tax assets	401	294
Other
Deferred tax assets —
Deferred income tax assets	786	731
Alabama Power
Deferred tax liabilities —
Deferred income tax liabilities	5,161	4,584
Deferred tax assets —
Deferred income tax assets	1,901	1,622
Valuation allowance	0	0
Net deferred income tax assets	1,901	1,622
Net deferred income taxes liabilities	3,260	2,962
Accumulated deferred income taxes – assets	0	0
Accumulated deferred income taxes – liabilities	3,260	2,962
Alabama Power | Accelerated depreciation
Deferred tax liabilities —
Deferred income tax liabilities	2,402	2,236
Alabama Power | Property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	912	865
Alabama Power | Federal effect of net state deferred tax liabilities
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Deferred tax assets —
Deferred income tax assets	162	155
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	242	149
Deferred tax assets —
Deferred income tax assets	334	286
Alabama Power | Premium on reacquired debt
Deferred tax liabilities —
Deferred income tax liabilities	13	14
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	311	260
Alabama Power | Regulatory assets associated with remaining book value of retired assets
Deferred tax liabilities —
Deferred income tax liabilities	174	6
Alabama Power | AROs
Deferred tax liabilities —
Deferred income tax liabilities	613	276
Alabama Power | AROs
Deferred tax liabilities —
Deferred income tax liabilities	360	607
Alabama Power | Other
Deferred tax liabilities —
Deferred income tax liabilities	134	171
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	11	11
Alabama Power | Alternative minimum tax carryforward
Deferred tax assets —
Deferred income tax assets		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	8	10
Alabama Power | AROs
Deferred tax assets —
Deferred income tax assets	973	883
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	240	130
Alabama Power | Other
Deferred tax assets —
Deferred income tax assets	173	147
Georgia Power
Deferred tax liabilities —
Deferred income tax liabilities	6,345	6,098
Deferred tax assets —
Deferred income tax assets	3,123	3,078
Valuation allowance	(35)	(42)
Net deferred income tax assets	3,088	3,036
Net deferred income taxes liabilities	3,257	3,062
Accumulated deferred income taxes – assets	0	0
Accumulated deferred income taxes – liabilities	3,257	3,062
Georgia Power | Accelerated depreciation
Deferred tax liabilities —
Deferred income tax liabilities	3,058	3,005
Georgia Power | Property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	643	633
Georgia Power | Federal effect of net state deferred tax liabilities
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Deferred tax assets —
Deferred income tax assets	63	71
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	351	290
Deferred tax assets —
Deferred income tax assets	488	444
Georgia Power | Premium on reacquired debt
Deferred tax liabilities —
Deferred income tax liabilities	70	74
Georgia Power | Storm damage reserves
Deferred tax liabilities —
Deferred income tax liabilities	109	111
Georgia Power | Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	403	344
Georgia Power | Regulatory assets associated with remaining book value of retired assets
Deferred tax liabilities —
Deferred income tax liabilities	159	39
Georgia Power | AROs
Deferred tax liabilities —
Deferred income tax liabilities	1,066	925
Georgia Power | AROs
Deferred tax liabilities —
Deferred income tax liabilities	405	575
Georgia Power | Other
Deferred tax liabilities —
Deferred income tax liabilities	81	102
Georgia Power | Other property basis differences
Deferred tax assets —
Deferred income tax assets	65	61
Georgia Power | ITC and PTC carryforward
Deferred tax assets —
Deferred income tax assets	435	430
Georgia Power | Alternative minimum tax carryforward
Deferred tax assets —
Deferred income tax assets		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	18	3
Georgia Power | AROs
Deferred tax assets —
Deferred income tax assets	1,471	1,500
Georgia Power | Estimated loss on plants under construction
Deferred tax assets —
Deferred income tax assets	283	283
Georgia Power | Other deferred state tax attributes
Deferred tax assets —
Deferred income tax assets	13	19
Georgia Power | Regulatory liability associated with the Tax Reform Legislation (not subject to normalization)
Deferred tax assets —
Deferred income tax assets	133	127
Georgia Power | Other
Deferred tax assets —
Deferred income tax assets	154	140
Mississippi Power
Deferred tax liabilities —
Deferred income tax liabilities	695	702
Deferred tax assets —
Deferred income tax assets	451	515
Valuation allowance	(41)	(41)
Net deferred income tax assets	410	474
Net deferred income taxes liabilities	285	228
Accumulated deferred income taxes – assets	(139)	(150)
Accumulated deferred income taxes – liabilities	424	378
Mississippi Power | Accelerated depreciation
Deferred tax liabilities —
Deferred income tax liabilities	315	335
Mississippi Power | Property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	143	162
Mississippi Power | Federal effect of net state deferred tax liabilities
Deferred tax liabilities —
Deferred income tax liabilities	24	36
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	38	25
Deferred tax assets —
Deferred income tax assets	72	62
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	55	45
Mississippi Power | Regulatory assets associated with remaining book value of retired assets
Deferred tax liabilities —
Deferred income tax liabilities	8	11
Mississippi Power | AROs
Deferred tax liabilities —
Deferred income tax liabilities	44	31
Mississippi Power | AROs
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Mississippi Power | Other
Deferred tax liabilities —
Deferred income tax liabilities	68	57
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 | Alternative minimum tax carryforward
Deferred tax assets —
Deferred income tax assets		32
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	44	31
Mississippi Power | Estimated loss on plants under construction
Deferred tax assets —
Deferred income tax assets	0	63
Mississippi Power | Other deferred state tax attributes
Deferred tax assets —
Deferred income tax assets	251	251
Mississippi Power | Regulatory liability associated with the Tax Reform Legislation (not subject to normalization)
Deferred tax assets —
Deferred income tax assets	28	29
Mississippi Power | Other
Deferred tax assets —
Deferred income tax assets	56	47
Southern Power
Deferred tax liabilities —
Deferred income tax liabilities	1,445	1,523
Deferred tax assets —
Deferred income tax assets	1,917	2,631
Valuation allowance	(36)	(27)
Net deferred income tax assets	1,881	2,604
Net deferred income taxes assets	(436)	(1,081)
Accumulated deferred income taxes – assets	(551)	(1,186)
Accumulated deferred income taxes – liabilities	115	105
Southern Power | Accelerated depreciation
Deferred tax liabilities —
Deferred income tax liabilities	1,422	1,483
Southern Power | Property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Southern Power | Federal effect of net state deferred tax liabilities
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Deferred tax assets —
Deferred income tax assets	24	22
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	6
Deferred tax assets —
Deferred income tax assets	5	7
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 | Regulatory assets associated with 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	11	34
Southern Power | Other property basis differences
Deferred tax assets —
Deferred income tax assets	146	172
Southern Power | ITC and PTC carryforward
Deferred tax assets —
Deferred income tax assets	1,445	2,128
Southern Power | Alternative minimum tax carryforward
Deferred tax assets —
Deferred income tax assets		21
Southern Power | Other partnership basis difference
Deferred tax assets —
Deferred income tax assets	169	162
Southern Power | Other comprehensive losses
Deferred tax assets —
Deferred income tax assets	10	0
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	72	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	46	47
Southern Company Gas
Deferred tax liabilities —
Deferred income tax liabilities	1,676	1,493
Deferred tax assets —
Deferred income tax assets	462	489
Valuation allowance	(5)	(12)
Net deferred income tax assets	457	477
Net deferred income taxes liabilities	1,219	1,016
Accumulated deferred income taxes – assets	0	0
Accumulated deferred income taxes – liabilities	1,219	1,016
Southern Company Gas | Accelerated depreciation
Deferred tax liabilities —
Deferred income tax liabilities	1,288	1,176
Southern Company Gas | Property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	133	134
Southern Company Gas | Federal effect of net state deferred tax liabilities
Deferred tax liabilities —
Deferred income tax liabilities	0	0
Deferred tax assets —
Deferred income tax assets	56	46
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	12	6
Deferred tax assets —
Deferred income tax assets	111	150
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 | Regulatory assets associated with 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	198	132
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 | Alternative minimum tax carryforward
Deferred tax assets —
Deferred income tax assets		0
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	8	0
Southern Company Gas | Regulatory liability associated with the Tax Reform Legislation (not subject to normalization)
Deferred tax assets —
Deferred income tax assets	0	8
Southern Company Gas | Other
Deferred tax assets —
Deferred income tax assets	$ 287	$ 285