Segment and Related Information - Financial Data for Business Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating revenues	$ 6,201	$ 6,264		$ 17,403	$ 14,715
Segment net income	1,069	1,139		347	2,251
Total assets	110,315			110,315		$ 109,697
Kemper IGCC
Segment Reporting Information [Line Items]
Pre-tax charge to income	34	88		3,200	222
After tax charge to income	21	54		2,200	137
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	(40)	(37)		(119)	(118)
Segment net income	2	(1)		0	(8)
Total assets	(1,532)			(1,532)		(1,624)
Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	5,523	5,619		14,239	13,979
Segment net income	1,132	1,198		276	2,401
Total assets	87,382			87,382		86,994
SOUTHERN Co GAS | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	565	543		2,841	543
Segment net income	15	4		303	4
Total assets	22,190			22,190		21,853
All Other | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	153	139		442	311
Segment net income	(80)	(62)		(232)	(146)
Total assets	2,275			2,275		2,474
SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Operating revenues	618	500		1,597	1,189
Segment net income	124	176		276	315
Total assets	14,648			14,648		15,169
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	565	543		2,841
Segment net income	15	4		303
Total assets	22,190			22,190		21,853
EBIT		4
SOUTHERN Co GAS | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	607	586		3,000
Segment net income	44			325
Total assets	24,052			24,052		24,875
EBIT		26
SOUTHERN Co GAS | Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(44)	(45)		(166)
Segment net income	0			0
Total assets	(13,262)			(13,262)		(14,167)
EBIT		0
SOUTHERN Co GAS | All Other | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	2	2		7
Segment net income	(29)			(22)
Total assets	11,400			11,400		11,145
EBIT		(22)
SOUTHERN Co GAS | Gas distribution operations | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	472	455		2,255
Segment net income	52			223
Total assets	18,711			18,711		19,453
EBIT		27
SOUTHERN Co GAS | Gas marketing services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	143	126		597
Segment net income	1			36
Total assets	2,089			2,089		2,084
EBIT		(4)
SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	(24)	(8)		95
Segment net income	(23)			28
Total assets	893			893		1,127
EBIT		(11)
Less Gross Gas Costs	1,538	1,773		5,048
SOUTHERN Co GAS | Gas Midstream Operations | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	16	13		53
Segment net income	14			38
Total assets	2,359			2,359		2,211
EBIT		14
Predecessor | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues			$ 1,905
Segment net income			131
EBIT			328
Predecessor | SOUTHERN Co GAS | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			2,003
EBIT			388
Predecessor | SOUTHERN Co GAS | Eliminations
Segment Reporting Information [Line Items]
Operating revenues			(102)
EBIT			0
Predecessor | SOUTHERN Co GAS | All Other | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			4
EBIT			(60)
Predecessor | SOUTHERN Co GAS | Gas distribution operations | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			1,575
EBIT			353
Predecessor | SOUTHERN Co GAS | Gas marketing services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			435
EBIT			109
Predecessor | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			(32)
EBIT			(68)
Less Gross Gas Costs			2,675
Predecessor | SOUTHERN Co GAS | Gas Midstream Operations | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			25
EBIT			(6)
Third Party Gross Revenues | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	1,411	1,688		4,781
Third Party Gross Revenues | Predecessor | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			2,500
Intercompany Revenues | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	103	77		362
Intercompany Revenues | Predecessor | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			143
Total Gross Revenues | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	1,514	1,765		5,143
Total Gross Revenues | Predecessor | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			$ 2,643
Reportable Subsegments | Traditional Electric Operating Companies | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	5,017	5,236		12,960	13,120
Segment net income	1,008	1,022		0	2,086
Total assets	73,056			73,056		72,141
Reportable Subsegments | SOUTHERN POWER CO | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	618	500		1,597	1,189
Segment net income	124	176		276	315
Total assets	14,648			14,648		15,169
Intersubsegment Eliminations | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	(112)	(117)		(318)	(330)
Segment net income	0	$ 0		0	$ 0
Total assets	$ (322)			$ (322)		$ (316)