Revenue from Contracts with Customers - Disaggregate Revenue Sources (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 6,372	$ 5,771
Other Income	90
Retail electric revenues
Disaggregation of Revenue [Line Items]
Lease income	18
Increase (decrease) in revenue from cost recovery mechanisms	117
Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	1,539
Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	1,243
Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	756
Other
Disaggregation of Revenue [Line Items]
Total operating revenues	30
Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	1,224
Alternative revenue programs(b)
Disaggregation of Revenue [Line Items]
Total operating revenues	(24)
Total retail electric and gas distribution revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	4,768
Wholesale energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	468
Revenue from transactions accounted for as derivatives	93
Lease income from PPA contracts	69
Wholesale capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	151
Lease income from PPA contracts	30
Other natural gas revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	407
Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	578
SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Total operating revenues	1,639	1,560
SOUTHERN Co GAS | Natural gas distribution revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	1,200
SOUTHERN Co GAS | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	660
SOUTHERN Co GAS | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	192
SOUTHERN Co GAS | Transportation
Disaggregation of Revenue [Line Items]
Total operating revenues	277
SOUTHERN Co GAS | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	17
SOUTHERN Co GAS | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	78
SOUTHERN Co GAS | Alternative revenue programs(b)
Disaggregation of Revenue [Line Items]
Total operating revenues	(24)
SOUTHERN Co GAS | Gas marketing services
Disaggregation of Revenue [Line Items]
Total operating revenues	271
Lease income	1
Revenue from transactions accounted for as derivatives	3
SOUTHERN Co GAS | Wholesale gas services
Disaggregation of Revenue [Line Items]
Total operating revenues	146
SOUTHERN Co GAS | Gas midstream operations
Disaggregation of Revenue [Line Items]
Total operating revenues	22
ALABAMA POWER CO
Disaggregation of Revenue [Line Items]
Total operating revenues	1,473	1,382
Other Income	25
ALABAMA POWER CO | Retail electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	1,285
Increase (decrease) in revenue from cost recovery mechanisms	47
ALABAMA POWER CO | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	570
ALABAMA POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	371
ALABAMA POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	338
ALABAMA POWER CO | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	6
ALABAMA POWER CO | Wholesale energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	101
Revenue from transactions accounted for as derivatives	5
ALABAMA POWER CO | Wholesale capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	24
ALABAMA POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	63
GEORGIA POWER CO
Disaggregation of Revenue [Line Items]
Total operating revenues	1,961	1,832
Other Income	26
GEORGIA POWER CO | Retail electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	1,798
Lease income	18
Increase (decrease) in revenue from cost recovery mechanisms	10
GEORGIA POWER CO | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	744
GEORGIA POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	717
GEORGIA POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	316
GEORGIA POWER CO | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	21
GEORGIA POWER CO | Wholesale energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	40
Revenue from transactions accounted for as derivatives	7
GEORGIA POWER CO | Wholesale capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	14
GEORGIA POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	109
Lease income	33
GULF POWER CO
Disaggregation of Revenue [Line Items]
Total operating revenues	348	350
Other Income	2
GULF POWER CO | Retail electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	290
Increase (decrease) in revenue from cost recovery mechanisms	(16)
GULF POWER CO | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	165
GULF POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	92
GULF POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	32
GULF POWER CO | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	1
GULF POWER CO | Wholesale energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	35
Revenue from transactions accounted for as derivatives	1
GULF POWER CO | Wholesale capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	6
GULF POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	17
MISSISSIPPI POWER CO
Disaggregation of Revenue [Line Items]
Total operating revenues	302	272
MISSISSIPPI POWER CO | Retail electric revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	194
Increase (decrease) in revenue from cost recovery mechanisms	76
MISSISSIPPI POWER CO | Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	60
MISSISSIPPI POWER CO | Commercial
Disaggregation of Revenue [Line Items]
Total operating revenues	62
MISSISSIPPI POWER CO | Industrial
Disaggregation of Revenue [Line Items]
Total operating revenues	70
MISSISSIPPI POWER CO | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	2
MISSISSIPPI POWER CO | Wholesale energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	93
Revenue from transactions accounted for as derivatives	1
MISSISSIPPI POWER CO | Wholesale capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	4
MISSISSIPPI POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	11
SOUTHERN POWER CO
Disaggregation of Revenue [Line Items]
Total operating revenues	509	450
SOUTHERN POWER CO | Other revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	2
SOUTHERN POWER CO | PPA capacity revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	138
Lease income from PPA contracts	47
SOUTHERN POWER CO | PPA energy revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	254
Revenue from transactions accounted for as derivatives	79
Lease income from PPA contracts	76
SOUTHERN POWER CO | Non-PPA revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	115
Operating Segments | SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Total operating revenues	1,671	1,624
Wholesale gas services | Operating Segments | SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Total operating revenues	166	131
Wholesale gas services | Third Party Gross Revenues | Operating Segments | SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Total operating revenues	1,938	$ 1,839
Revenue from transactions accounted for as derivatives	$ 1,100