Revenue from Contracts with Customers - Disaggregate Revenue Sources (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 6,159		$ 18,158
Total operating revenues	6,159	$ 6,201	18,158	$ 17,403
Other Income	92		274
Retail electric revenues
Disaggregation of Revenue [Line Items]
Leases revenues	17		54
Increase (decrease) in revenue from cost recovery mechanisms	(98)		4
Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	2,148		5,266
Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	1,527		4,084
Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	901		2,471
Other
Disaggregation of Revenue [Line Items]
Total operating revenues	29		92
Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	433		2,299
Alternative revenue programs
Disaggregation of Revenue [Line Items]
Total operating revenues	5		(23)
Total retail electric and gas distribution revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	5,043		14,189
Wholesale energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	516		1,444
Revenue from transactions accounted for as derivatives	63		217
Lease income from PPA contracts	130		318
Wholesale capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	177		479
Lease income from PPA contracts	31		92
Other natural gas revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	54		530
Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	369		1,516
Total operating revenues	199	168	1,007	494
SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Total operating revenues	492		2,861
Total operating revenues	492	565	2,861	2,841
SOUTHERN Co GAS | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	438		2,276
SOUTHERN Co GAS | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	149		1,082
SOUTHERN Co GAS | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	45		313
SOUTHERN Co GAS | Transportation
Disaggregation of Revenue [Line Items]
Total operating revenues	203		708
SOUTHERN Co GAS | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	4		28
SOUTHERN Co GAS | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	32		168
SOUTHERN Co GAS | Alternative revenue programs
Disaggregation of Revenue [Line Items]
Total operating revenues	5		(23)
SOUTHERN Co GAS | Gas marketing Services
Disaggregation of Revenue [Line Items]
Total operating revenues	44		403
Other Income			4
SOUTHERN Co GAS | Wholesale gas services
Disaggregation of Revenue [Line Items]
Total operating revenues	(10)		121
SOUTHERN Co GAS | Gas midstream operations
Disaggregation of Revenue [Line Items]
Total operating revenues	20		60
SOUTHERN Co GAS | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0		1
Total operating revenues	0	33	55	95
ALABAMA POWER CO
Disaggregation of Revenue [Line Items]
Total operating revenues	1,740		4,716
Total operating revenues	1,740	1,740	4,716	4,606
Other Income	27		79
ALABAMA POWER CO | Retail electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	1,584		4,208
Increase (decrease) in revenue from cost recovery mechanisms	(12)		113
ALABAMA POWER CO | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	721		1,848
ALABAMA POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	464		1,238
ALABAMA POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	392		1,103
ALABAMA POWER CO | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	7		19
ALABAMA POWER CO | Wholesale energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	62		234
Revenue from transactions accounted for as derivatives	6		14
ALABAMA POWER CO | Wholesale capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	26		75
ALABAMA POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	68		199
Total operating revenues	68	50	199	158
GEORGIA POWER CO
Disaggregation of Revenue [Line Items]
Total operating revenues	2,593		6,601
Total operating revenues	2,593	2,546	6,601	6,426
Other Income	28		80
GEORGIA POWER CO | Retail electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	2,425		6,112
Leases revenues	17		54
Increase (decrease) in revenue from cost recovery mechanisms	(47)		(35)
GEORGIA POWER CO | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	1,142		2,671
GEORGIA POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	877		2,343
GEORGIA POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	385		1,036
GEORGIA POWER CO | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	21		62
GEORGIA POWER CO | Wholesale energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	33		99
Revenue from transactions accounted for as derivatives	8		21
GEORGIA POWER CO | Wholesale capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	14		41
GEORGIA POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	121		349
Leases revenues	34		100
Total operating revenues	121	93	349	284
GULF POWER CO
Disaggregation of Revenue [Line Items]
Total operating revenues	414		1,106
Total operating revenues	414	437	1,106	1,144
Other Income	2		5
GULF POWER CO | Retail electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	341		932
Increase (decrease) in revenue from cost recovery mechanisms	(36)		(63)
GULF POWER CO | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	200		537
GULF POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	103		291
GULF POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	37		100
GULF POWER CO | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	1		4
GULF POWER CO | Wholesale energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	48		104
GULF POWER CO | Wholesale capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	7		20
GULF POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	18		50
Total operating revenues	18	20	50	53
MISSISSIPPI POWER CO
Disaggregation of Revenue [Line Items]
Total operating revenues	358		956
Total operating revenues	358	341	956	915
MISSISSIPPI POWER CO | Retail electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	254		660
Increase (decrease) in revenue from cost recovery mechanisms	(3)		(11)
MISSISSIPPI POWER CO | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	85		209
MISSISSIPPI POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	82		212
MISSISSIPPI POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	86		233
MISSISSIPPI POWER CO | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	1		6
MISSISSIPPI POWER CO | Wholesale energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	92		259
MISSISSIPPI POWER CO | Wholesale capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	1		6
MISSISSIPPI POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	11		31
Total operating revenues	11	5	31	14
SOUTHERN POWER CO
Disaggregation of Revenue [Line Items]
Total operating revenues	635		1,699
Total operating revenues	635	618	1,699	1,597
SOUTHERN POWER CO | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	168		450
Lease income from PPA contracts	47		141
SOUTHERN POWER CO | PPA energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	336		892
Revenue from transactions accounted for as derivatives	47		176
Lease income from PPA contracts	139		342
SOUTHERN POWER CO | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	126		347
SOUTHERN POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	5		10
Total operating revenues	5	3	10	9
Operating Segments | SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Total operating revenues	497	607	2,902	3,000
Wholesale gas services | Operating Segments | SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Total operating revenues	(8)	(24)	142	95
Wholesale gas services | Third Party Gross Revenues | Operating Segments | SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Revenue from transactions accounted for as derivatives	900		2,700
Total operating revenues	$ 1,573	$ 1,411	$ 4,847	$ 4,781