Segment and Related Information - Financial Data for Business Segments (Details) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2018 USD ($) utility	Mar. 31, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) utility	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Operating revenues	$ 5,627		$ 5,430	$ 11,999	$ 11,202
Segment net income	(154)		(1,381)	784	(723)
Goodwill	5,315			5,315		$ 6,268
Total assets	112,777			112,777		111,005
Goodwill impairment				42
Plant Scherer Unit 3
Segment Reporting Information [Line Items]
After tax charge to income					20
Loss on Plant Scherer Unit 3					33
Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(49)		(37)	(100)	(79)
Segment net income	3		(2)	4	0
Goodwill	0			0		0
Total assets	(1,791)			(1,791)		(1,619)
Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	4,565		4,585	8,948	8,716
Segment net income	(26)		(1,360)	706	(859)
Goodwill	2			2		2
Total assets	88,749			88,749		87,085
SOUTHERN Co GAS | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	730		716	2,369	2,276
Segment net income	(31)		49	248	288
Goodwill	5,015			5,015		5,967
Total assets	22,112			22,112		22,987
All Other | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	381		166	782	289
Segment net income	(100)		(68)	(174)	(152)
Goodwill	298			298		299
Total assets	3,707			3,707		2,552
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	1,060		3,000	1,100	3,100
Public utilities, property, plant and equipment, amount of loss (recovery) on plant abandonment after tax	800		2,100	800	2,200
SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Operating revenues	555		529	1,064	979
Segment net income	22		82	143	151
Total assets	15,428			15,428		15,206
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	730		716	2,369	2,276
Segment net income	(31)		49	248	288
Goodwill	5,015			5,015		5,967
Total assets	22,112			22,112		22,987
Goodwill impairment	0	$ 42	0	42	0
Loss on disposition	36		0	36	0
SOUTHERN Co GAS | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	734		769	2,405	2,393
Segment net income	(15)		50	274	282
Total assets	23,351			23,351		24,842
SOUTHERN Co GAS | Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(5)		(56)	(38)	(122)
Segment net income	0		0	0	0
Total assets	(12,783)			(12,783)		(14,039)
SOUTHERN Co GAS | All Other | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	1		3	2	5
Segment net income	(16)		(1)	(26)	6
Total assets	11,544			11,544		12,184
SOUTHERN Co GAS | Gas Distribution Operations
Segment Reporting Information [Line Items]
Goodwill	4,034			4,034		4,702
Goodwill impairment				0
SOUTHERN Co GAS | Gas Distribution Operations | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	643		603	1,856	1,783
Segment net income	68		54	216	171
Total assets	18,654			18,654		19,358
Revenue on disposition	70		66
Net income on disposition				237	224
SOUTHERN Co GAS | Gas Marketing Services
Segment Reporting Information [Line Items]
Goodwill	981			981		1,265
Goodwill impairment				42
SOUTHERN Co GAS | Gas Marketing Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	89		166	359	454
Segment net income	(76)		4	(63)	35
Total assets	1,610			1,610		2,147
Revenue on disposition	24		32
Net income on disposition				55	63
Loss on disposition	36			36
SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	(16)		(12)	150	119
Segment net income	(21)		(17)	83	51
Total assets	818			818		1,096
Less Gross Gas Costs	1,454		1,666	3,393	3,510
SOUTHERN Co GAS | Gas Midstream Operations | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	18		12	40	37
Segment net income	14		9	38	25
Total assets	2,269			2,269		2,241
Third Party Gross Revenues | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	1,336		1,531	3,274	3,370
Intercompany Revenues | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	102		123	269	259
Total Gross Revenues | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	1,438		1,654	3,543	3,629
Reportable Subsegments | Traditional Electric Operating Companies | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	4,124		4,157	8,104	7,943
Segment net income	(48)		(1,442)	563	(1,010)
Goodwill	0			0		0
Total assets	73,634			73,634		72,204
Reportable Subsegments | SOUTHERN POWER CO | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	555		529	1,064	979
Segment net income	22		82	143	151
Goodwill	2			2		2
Total assets	15,428			15,428		15,206
Intersubsegment Eliminations | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	(114)		(101)	(220)	(206)
Segment net income	0		$ 0	0	$ 0
Goodwill	0			0		0
Total assets	(313)			(313)		$ (325)
Disposal Group, Held-for-sale, Not Discontinued Operations | Florida Plants | SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Pre-tax impairment charge	119			119
After tax impairment charge	$ 89			$ 88
Disposal Group, Disposed of by Sale, Not Discontinued Operations | SOUTHERN Co GAS | Gas Distribution Operations | Operating Segments
Segment Reporting Information [Line Items]
Number of natural gas distribution utilities disposed | utility	3			3
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Pivotal Home Solutions | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Goodwill impairment		$ 42		$ 42