Segment and Related Information - Financial Data for Business Segments (Details) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018 USD ($) utility	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) utility	Sep. 30, 2017 USD ($)	Jul. 31, 2018 utility	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Operating revenues	$ 6,159		$ 6,201	$ 18,158	$ 17,403
Segment net income (loss)	1,164		1,069	1,948	347
Goodwill	5,315			5,315			$ 6,268
Total assets	111,717			111,717			111,005
Pre-tax impairment charges	36		0	197	0
Net gain (loss) on dispositions	353		0	317	19
Goodwill impairment				42
Plant Scherer Unit 3
Segment Reporting Information [Line Items]
Loss on Plant Scherer Unit 3					33
After tax charge to income					20
GEORGIA POWER AND MISSISSIPPI POWER CO | Kemper County Energy Facility
Segment Reporting Information [Line Items]
Public utilities, property, plant and equipment, amount of loss (recovery) on plant abandonment	1		34	1,100	3,200
Public utilities, property, plant and equipment, amount of loss (recovery) on plant abandonment after tax	1		21	800	2,200
SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Operating revenues	635		618	1,699	1,597
Segment net income (loss)	92		124	235	276
Total assets	15,355			15,355			15,206
Pre-tax impairment charges	36		0	155	0
SOUTHERN POWER CO | Wind Generating Facility
Segment Reporting Information [Line Items]
Pre-tax impairment charges	36			155
After tax impairment charges	27			116
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	492		565	2,861	2,841
Segment net income (loss)	46		15	294	303
Goodwill	5,015			5,015			5,967
Total assets	20,398			20,398			22,987
Net gain (loss) on dispositions	353		0	317	0
Goodwill impairment	0	$ 42	0	42	0
SOUTHERN Co GAS | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Segment Reporting Information [Line Items]
Number of natural gas distribution utilities disposed | utility						3
SOUTHERN Co GAS | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Pivotal Home Solutions
Segment Reporting Information [Line Items]
Net gain (loss) on dispositions	353			317
Net gain (loss) on dispositions, net of tax	40			(35)
Goodwill impairment		$ 42		42
Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	5,509		5,523	14,456	14,239
Segment net income (loss)	1,240		1,132	1,946	276
Goodwill	2			2			2
Total assets	90,102			90,102			87,085
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	492		565	2,861	2,841
Gas Distribution Operations | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Goodwill	4,034			4,034			4,702
Net gain (loss) on dispositions	351			351
Net gain (loss) on dispositions, net of tax	38			38
Goodwill impairment				0
Gas Marketing Services | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Goodwill	981			981			1,265
Net gain (loss) on dispositions	2			(34)
Net gain (loss) on dispositions, net of tax	2			(73)
Goodwill impairment				42
Operating Segments | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	497		607	2,902	3,000
Segment net income (loss)	64		44	338	325
Total assets	21,524			21,524			24,842
Operating Segments | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	492		565	2,861	2,841
Segment net income (loss)	46		15	294	303
Goodwill	5,015			5,015			5,967
Total assets	20,398			20,398			22,987
Operating Segments | Gas Distribution Operations | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	441		472	2,297	2,255
Segment net income (loss)	74		52	290	223
Total assets	16,850			16,850			19,358
Revenue on disposition	$ 8		50
Net income on disposition				$ 245	274
Operating Segments | Gas Distribution Operations | SOUTHERN Co GAS | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Segment Reporting Information [Line Items]
Number of natural gas distribution utilities disposed | utility	3			3
Operating Segments | Gas Marketing Services | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	$ 44		143	$ 403	597
Segment net income (loss)	(8)		1	(71)	36
Total assets	1,522			1,522			2,147
Revenue on disposition			32
Net income on disposition				55	95
Operating Segments | Wholesale Gas Services | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	(8)		(24)	142	95
Segment net income (loss)	(18)		(23)	65	28
Total assets	855			855			1,096
Less Gross Gas Costs	1,663		1,538	5,057	5,048
Operating Segments | Gas Midstream Operations | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	20		16	60	53
Segment net income (loss)	16		14	54	38
Total assets	2,297			2,297			2,241
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	202		153	984	442
Segment net income (loss)	(119)		(80)	(292)	(232)
Goodwill	298			298			299
Total assets	3,086			3,086			2,552
Operating Segments | All Other | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	1		2	3	7
Segment net income (loss)	(18)		(29)	(44)	(22)
Total assets	10,146			10,146			12,184
Operating Segments | Third Party Gross Revenues | Wholesale Gas Services | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	1,573		1,411	4,847	4,781
Operating Segments | Intercompany Revenues | Wholesale Gas Services | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	82		103	352	362
Operating Segments | Total Gross Revenues | Wholesale Gas Services | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	1,655		1,514	5,199	5,143
Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(44)		(40)	(143)	(119)
Segment net income (loss)	(3)		2	0	0
Goodwill	0			0			0
Total assets	(1,869)			(1,869)			(1,619)
Eliminations | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	(6)		(44)	(44)	(166)
Segment net income (loss)	0		0	0	0
Total assets	(11,272)			(11,272)			(14,039)
Reportable Subsegments | Traditional Electric Operating Companies | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	5,014		5,017	13,117	12,960
Segment net income (loss)	1,148		1,008	1,711	0
Goodwill	0			0			0
Total assets	75,069			75,069			72,204
Reportable Subsegments | SOUTHERN POWER CO | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	635		618	1,699	1,597
Segment net income (loss)	92		124	235	276
Goodwill	2			2			2
Total assets	15,355			15,355			15,206
Intersubsegment Eliminations | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	(140)		(112)	(360)	(318)
Segment net income (loss)	0		$ 0	0	$ 0
Goodwill	0			0			0
Total assets	$ (322)			$ (322)			$ (325)