Segment and Related Information - Financial Data for Business Segments (Details) $ in Millions	3 Months Ended
Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($) utility	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Operating revenues	$ 5,412	$ 6,372
Segment net income (loss)	2,084	938
Goodwill	5,284		$ 5,315
Total assets	114,096		116,914
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	1,474	1,639
Segment net income (loss)	270	279
Goodwill	5,015		5,015
Total assets	20,952		21,448
Goodwill impairment	0	$ 42
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
Traditional Electric Operating Companies \| 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	2	$ 44
Public utilities, property, plant and equipment, amount of loss (recovery) on plant abandonment after tax	1	33
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	1,474	1,639
SOUTHERN Co GAS \| Disposal Group, Disposed of by Sale, Not Discontinued Operations \| Pivotal Home Solutions
Segment Reporting Information [Line Items]
Goodwill impairment		42
All Other \| Disposal Group, Disposed of by Sale, Not Discontinued Operations \| GULF POWER CO
Segment Reporting Information [Line Items]
Preliminary pre-tax gain	2,500
Preliminary after-tax gain	1,300
Gas Distribution Operations \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Goodwill	4,034
Gas Distribution Operations \| SOUTHERN Co GAS \| Disposal Group, Disposed of by Sale, Not Discontinued Operations
Segment Reporting Information [Line Items]
Operating revenues		167
Gas Marketing Services \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Goodwill	981
Operating revenues		32
Operating Segments \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	1,495	1,657
Segment net income (loss)	273	293
Total assets	21,592		21,918
Operating Segments \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	1,474	1,639
Segment net income (loss)	270	279
Goodwill	5,015		5,015
Total assets	20,952		21,448
Operating Segments \| All Other
Segment Reporting Information [Line Items]
Operating revenues	182	401
Segment net income (loss)	1,195	(74)
Goodwill	268		298
Total assets	3,391		3,285
Operating Segments \| All Other \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	11	15
Segment net income (loss)	(3)	(14)
Total assets	10,900		11,112
Operating Segments \| Gas Distribution Operations \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	1,172	1,212
Segment net income (loss)	133	149
Total assets	17,379		17,266
Operating Segments \| Gas Pipeline Investments \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	8	8
Segment net income (loss)	32	27
Total assets	1,781		1,763
Operating Segments \| Wholesale Gas Services \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	86	166
Segment net income (loss)	47	104
Total assets	821		1,302
Less Gross Gas Costs	1,928	1,939
Operating Segments \| Wholesale Gas Services \| SOUTHERN Co GAS \| Third Party Gross Revenues
Segment Reporting Information [Line Items]
Operating revenues	1,926	1,938
Operating Segments \| Wholesale Gas Services \| SOUTHERN Co GAS \| Intercompany Revenues
Segment Reporting Information [Line Items]
Operating revenues	88	167
Operating Segments \| Wholesale Gas Services \| SOUTHERN Co GAS \| Total Gross Revenues
Segment Reporting Information [Line Items]
Operating revenues	2,014	2,105
Operating Segments \| Gas Marketing Services \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	229	271
Segment net income (loss)	61	13
Total assets	1,611		1,587
Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(39)	(50)
Segment net income (loss)	(2)	0
Goodwill	(1)		0
Total assets	(1,370)		(1,778)
Eliminations \| SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	(32)	(33)
Segment net income (loss)	0	0
Total assets	(11,540)		(11,582)
Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	3,795	4,382
Segment net income (loss)	621	733
Goodwill	2		2
Total assets	91,123		93,959
Electric Utilities \| Operating Segments \| Traditional Electric Operating Companies
Segment Reporting Information [Line Items]
Operating revenues	3,445	3,979
Segment net income (loss)	565	612
Goodwill	0		0
Total assets	76,798		79,382
Electric Utilities \| Operating Segments \| SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Operating revenues	443	509
Segment net income (loss)	56	121
Goodwill	2		2
Total assets	15,104		14,883
Electric Utilities \| Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(93)	(106)
Segment net income (loss)	0	$ 0
Goodwill	0		0
Total assets	$ (779)		$ (306)

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