Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred tax liabilities —
Deferred income tax liabilities	$ 15,011	$ 23,512
Deferred tax assets —
Deferred income tax assets	8,424	9,495
Regulatory liability associated with Tax Reform Legislation (not subject to normalization)	465	0
Valuation allowance	(149)	(23)
Deferred tax liabilities, net	6,736	14,040
Accumulated deferred income taxes – liability	(6,842)	(14,092)
Portion included in accumulated deferred tax assets	(106)	(52)
Accumulated deferred income taxes	6,842	14,092
Accelerated depreciation
Deferred tax liabilities —
Deferred income tax liabilities	10,267	15,392
Property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	955	2,708
Leveraged lease basis differences
Deferred tax liabilities —
Deferred income tax liabilities	251	314
Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	516	737
Deferred tax assets —
Deferred income tax assets	1,307	1,868
Premium on reacquired debt
Deferred tax liabilities —
Deferred income tax liabilities	54	89
Regulatory assets associated with employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	1,046	1,584
Regulatory assets associated with AROs
Deferred tax liabilities —
Deferred income tax liabilities	1,225	1,781
Federal effect of state deferred taxes
Deferred tax assets —
Deferred income tax assets	326	597
Over recovered fuel clause
Deferred tax assets —
Deferred income tax assets	0	66
Other property basis differences
Deferred tax assets —
Deferred income tax assets	446	401
Deferred costs
Deferred tax assets —
Deferred income tax assets	69	100
ITC carryforward
Deferred tax assets —
Deferred income tax assets	2,420	1,974
Federal NOL carryforward
Deferred tax assets —
Deferred income tax assets	518	1,084
Unbilled revenue
Deferred tax assets —
Deferred income tax assets	57	92
Other comprehensive losses
Deferred tax assets —
Deferred income tax assets	84	152
AROs
Deferred tax assets —
Deferred income tax assets	1,197	1,732
Estimated Loss on Kemper IGCC
Deferred tax assets —
Deferred income tax assets	722	484
Deferred state tax assets
Deferred tax assets —
Deferred income tax assets	328	266
Other deferred tax assets
Deferred tax assets —
Deferred income tax assets	485	679
Other deferred tax liabilities
Deferred tax liabilities —
Deferred income tax liabilities	697	907
ALABAMA POWER CO
Deferred tax liabilities —
Deferred income tax liabilities	3,788	6,198
Deferred tax assets —
Deferred income tax assets	1,028	1,544
Deferred tax liabilities, net	2,760	4,654
Accumulated deferred income taxes – liability	(2,760)	(4,654)
ALABAMA POWER CO | Accelerated depreciation
Deferred tax liabilities —
Deferred income tax liabilities	2,336	4,307
ALABAMA POWER CO | Property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	398	456
ALABAMA POWER CO | Leveraged lease basis differences
Deferred tax liabilities —
Deferred income tax liabilities	16	26
ALABAMA POWER CO | Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	162	201
Deferred tax assets —
Deferred income tax assets	286	427
ALABAMA POWER CO | Regulatory assets associated with employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	260	393
ALABAMA POWER CO | Regulatory assets associated with AROs
Deferred tax liabilities —
Deferred income tax liabilities	249	347
ALABAMA POWER CO | Federal effect of state deferred taxes
Deferred tax assets —
Deferred income tax assets	143	266
ALABAMA POWER CO | Unbilled revenue
Deferred tax assets —
Deferred income tax assets	22	36
ALABAMA POWER CO | Other comprehensive losses
Deferred tax assets —
Deferred income tax assets	10	19
ALABAMA POWER CO | Other deferred tax assets
Deferred tax assets —
Deferred income tax assets	93	139
ALABAMA POWER CO | Other deferred tax liabilities
Deferred tax liabilities —
Deferred income tax liabilities	147	179
ALABAMA POWER CO | AROs
Deferred tax liabilities —
Deferred income tax liabilities	220	289
Deferred tax assets —
Deferred income tax assets	469	636
ALABAMA POWER CO | Storm damage reserves
Deferred tax assets —
Deferred income tax assets	5	21
GEORGIA POWER CO
Deferred tax liabilities —
Deferred income tax liabilities	5,061	8,382
Deferred tax assets —
Deferred income tax assets	1,886	2,382
Regulatory liability associated with Tax Reform Legislation (not subject to normalization)	123	0
Deferred tax liabilities, net	3,175	6,000
Accumulated deferred income taxes – liability	(3,175)	(6,000)
GEORGIA POWER CO | Accelerated depreciation
Deferred tax liabilities —
Deferred income tax liabilities	3,540	5,266
GEORGIA POWER CO | Property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	0	957
GEORGIA POWER CO | Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	287	428
Deferred tax assets —
Deferred income tax assets	423	661
GEORGIA POWER CO | Premium on reacquired debt
Deferred tax liabilities —
Deferred income tax liabilities	34	56
GEORGIA POWER CO | Regulatory assets associated with employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	348	546
GEORGIA POWER CO | Regulatory assets associated with AROs
Deferred tax liabilities —
Deferred income tax liabilities	501	726
GEORGIA POWER CO | Federal effect of state deferred taxes
Deferred tax assets —
Deferred income tax assets	72	173
GEORGIA POWER CO | Other property basis differences
Deferred tax assets —
Deferred income tax assets	92	105
GEORGIA POWER CO | Deferred costs
Deferred tax assets —
Deferred income tax assets	69	100
GEORGIA POWER CO | Unbilled revenue
Deferred tax assets —
Deferred income tax assets	26	47
GEORGIA POWER CO | Other deferred tax assets
Deferred tax assets —
Deferred income tax assets	30	70
GEORGIA POWER CO | Regulatory Assets Associated With Storm Damage Reserves
Deferred tax liabilities —
Deferred income tax liabilities	89	83
GEORGIA POWER CO | Regulatory Assets Associated With Retired Assets
Deferred tax liabilities —
Deferred income tax liabilities	30	55
GEORGIA POWER CO | Other deferred tax liabilities
Deferred tax liabilities —
Deferred income tax liabilities	100	83
GEORGIA POWER CO | Regulatory Liabilities Associated With Asset Retirement Obligations
Deferred tax assets —
Deferred income tax assets	5	33
GEORGIA POWER CO | AROs
Deferred tax liabilities —
Deferred income tax liabilities	132	182
Deferred tax assets —
Deferred income tax assets	631	908
GEORGIA POWER CO | Tax credit carryforward
Deferred tax assets —
Deferred income tax assets	318	201
GEORGIA POWER CO | Federal Tax Credit Carryforward
Deferred tax assets —
Deferred income tax assets	97	84
GULF POWER CO
Deferred tax liabilities —
Deferred income tax liabilities	812	1,192
Deferred tax assets —
Deferred income tax assets	275	244
Deferred tax liabilities, net	537	948
Accumulated deferred income taxes – liability	(537)	(948)
GULF POWER CO | Accelerated depreciation
Deferred tax liabilities —
Deferred income tax liabilities	552	834
GULF POWER CO | Property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	105	123
GULF POWER CO | Employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	38	58
GULF POWER CO | Regulatory assets associated with employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	44	65
GULF POWER CO | Regulatory assets associated with AROs
Deferred tax liabilities —
Deferred income tax liabilities	38	55
GULF POWER CO | Federal effect of state deferred taxes
Deferred tax assets —
Deferred income tax assets	25	37
GULF POWER CO | Other property basis differences
Deferred tax assets —
Deferred income tax assets	98	1
Regulatory liability associated with Tax Reform Legislation (not subject to normalization)	19	0
GULF POWER CO | Other deferred tax assets
Deferred tax assets —
Deferred income tax assets	12	18
GULF POWER CO | Regulatory Assets
Deferred tax liabilities —
Deferred income tax liabilities	22	45
GULF POWER CO | Other deferred tax liabilities
Deferred tax liabilities —
Deferred income tax liabilities	13	12
GULF POWER CO | Pension and other employee benefits
Deferred tax assets —
Deferred income tax assets	49	72
GULF POWER CO | AROs
Deferred tax assets —
Deferred income tax assets	38	55
GULF POWER CO | Other postretirement benefit plans
Deferred tax assets —
Deferred income tax assets	17	26
GULF POWER CO | Tax credit carryforward
Deferred tax assets —
Deferred income tax assets	7	18
GULF POWER CO | Property damage reserves-liability
Deferred tax assets —
Deferred income tax assets	10	17
MISSISSIPPI POWER CO
Deferred tax liabilities —
Deferred income tax liabilities	804	1,778
Deferred tax assets —
Deferred income tax assets	1,173	1,022
Regulatory liability associated with Tax Reform Legislation (not subject to normalization)	375
Valuation allowance	(122)	0
Deferred tax assets, net	247
Deferred tax liabilities, net		756
Accumulated deferred income taxes – liability	0	(756)
MISSISSIPPI POWER CO | Accelerated depreciation
Deferred tax liabilities —
Deferred income tax liabilities	373	386
MISSISSIPPI POWER CO | Property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	242	852
Deferred tax assets —
Deferred income tax assets	70	0
MISSISSIPPI POWER CO | Regulatory assets associated with employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	45	70
MISSISSIPPI POWER CO | Regulatory assets associated with AROs
Deferred tax liabilities —
Deferred income tax liabilities	34	72
MISSISSIPPI POWER CO | Federal effect of state deferred taxes
Deferred tax liabilities —
Deferred income tax liabilities	9	0
Deferred tax assets —
Deferred income tax assets	0	19
MISSISSIPPI POWER CO | Over recovered fuel clause
Deferred tax assets —
Deferred income tax assets	0	26
MISSISSIPPI POWER CO | Estimated Loss on Kemper IGCC
Deferred tax assets —
Deferred income tax assets	722	484
MISSISSIPPI POWER CO | Other deferred tax assets
Deferred tax assets —
Deferred income tax assets	32	31
MISSISSIPPI POWER CO | Other deferred tax liabilities
Deferred tax liabilities —
Deferred income tax liabilities	11	91
MISSISSIPPI POWER CO | NOL State Carryforward
Deferred tax assets —
Deferred income tax assets	133	113
MISSISSIPPI POWER CO | Ad Valorem Over Under Recovery
Deferred tax liabilities —
Deferred income tax liabilities	11	14
MISSISSIPPI POWER CO | Regulatory Assets For Standard Compliance
Deferred tax liabilities —
Deferred income tax liabilities	11	8
MISSISSIPPI POWER CO | Deferred Federal Tax Assets
Deferred tax assets —
Deferred income tax assets	0	31
MISSISSIPPI POWER CO | Pension and other employee benefits
Deferred tax liabilities —
Deferred income tax liabilities	28	49
Deferred tax assets —
Deferred income tax assets	62	96
MISSISSIPPI POWER CO | Federal NOL
Deferred tax assets —
Deferred income tax assets	40	109
MISSISSIPPI POWER CO | Regulatory assets associated with the Kemper County energy facility
Deferred tax liabilities —
Deferred income tax liabilities	31	82
MISSISSIPPI POWER CO | Regulatory Assets Associated with Plant Daniel [Member]
Deferred tax liabilities —
Deferred income tax liabilities	9	13
MISSISSIPPI POWER CO | AROs
Deferred tax assets —
Deferred income tax assets	34	72
MISSISSIPPI POWER CO | Rate differential
Deferred tax liabilities —
Deferred income tax liabilities	0	141
MISSISSIPPI POWER CO | Affirmative Adjustments [Member]
Deferred tax assets —
Deferred income tax assets	31	0
MISSISSIPPI POWER CO | Unprotected Tax Reform Change [Member]
Deferred tax assets —
Deferred income tax assets	27	0
MISSISSIPPI POWER CO | Property insurance
Deferred tax assets —
Deferred income tax assets	15	27
MISSISSIPPI POWER CO | Premium on long-term debt
Deferred tax assets —
Deferred income tax assets	7	14
SOUTHERN POWER CO
Deferred tax liabilities —
Deferred income tax liabilities	1,954	2,495
Deferred tax assets —
Deferred income tax assets	2,693	2,937
Valuation allowance	(13)	0
Total, net of valuation allowances	2,680	2,937
Deferred tax assets, net	726	442
Accumulated deferred income taxes – liability	(199)	(152)
SOUTHERN POWER CO | Federal effect of state deferred taxes
Deferred tax assets —
Deferred income tax assets	42	53
SOUTHERN POWER CO | ITC carryforward
Deferred tax assets —
Deferred income tax assets	184	292
SOUTHERN POWER CO | Deferred state tax assets
Deferred tax assets —
Deferred income tax assets	77	60
SOUTHERN POWER CO | Other deferred tax assets
Deferred tax assets —
Deferred income tax assets	10	8
SOUTHERN POWER CO | Other deferred tax liabilities
Deferred tax liabilities —
Deferred income tax liabilities	6	27
SOUTHERN POWER CO | Accelerated depreciation and other property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	1,922	2,440
SOUTHERN POWER CO | Levelized capacity revenues
Deferred tax liabilities —
Deferred income tax liabilities	26	28
SOUTHERN POWER CO | Tax credit carryforward
Deferred tax assets —
Deferred income tax assets	21	15
SOUTHERN POWER CO | Unrealized Tax Credits
Deferred tax assets —
Deferred income tax assets	2,002	1,685
SOUTHERN POWER CO | Federal Net Operating Loss
Deferred tax assets —
Deferred income tax assets	333	808
SOUTHERN POWER CO | Investment In Partnerships
Deferred tax assets —
Deferred income tax assets	24	16
SOUTHERN Co GAS
Deferred tax liabilities —
Deferred income tax liabilities	1,927	2,554
Deferred tax assets —
Deferred income tax assets	849	598
Valuation allowance	(11)	(19)
Total, net of valuation allowances	838	579
Deferred tax liabilities, net	1,089	1,975
Accumulated deferred income taxes – liability	(1,089)	(1,975)
SOUTHERN Co GAS | Accelerated depreciation
Deferred tax liabilities —
Deferred income tax liabilities	1,436	1,954
SOUTHERN Co GAS | Property basis differences
Deferred tax liabilities —
Deferred income tax liabilities	204	311
SOUTHERN Co GAS | Employee benefit obligations
Deferred tax assets —
Deferred income tax assets	185	165
SOUTHERN Co GAS | Regulatory Liability Associated With Tax Reform [Member]
Deferred tax assets —
Deferred income tax assets	295	0
SOUTHERN Co GAS | Regulatory assets associated with employee benefit obligations
Deferred tax liabilities —
Deferred income tax liabilities	79	125
SOUTHERN Co GAS | Federal effect of state deferred taxes
Deferred tax assets —
Deferred income tax assets	54	42
SOUTHERN Co GAS | Other deferred tax assets
Deferred tax assets —
Deferred income tax assets	223	332
SOUTHERN Co GAS | Other deferred tax liabilities
Deferred tax liabilities —
Deferred income tax liabilities	208	164
SOUTHERN Co GAS | Federal NOL
Deferred tax assets —
Deferred income tax assets	92	59
Accumulated Deferred Income Taxes Assets | SOUTHERN POWER CO
Deferred tax assets —
Deferred tax assets, net	925	594
Accumulated Deferred Income Taxes Liability | SOUTHERN POWER CO
Deferred tax assets —
Accumulated deferred income taxes – liability	$ (199)	$ (152)