Segment and Related Information - Financial Data for Business Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Operating revenues	$ 6,372	$ 5,771
Segment net income	938	658
Total assets	111,567		$ 111,005
Kemper IGCC
Segment Reporting Information [Line Items]
Pre-tax charge to income	44	108
After tax charge to income	33	67
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	(50)	(43)
Segment net income	0	1
Total assets	(1,547)		(1,619)
Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	4,382	4,131
Segment net income	733	502
Total assets	87,813		87,085
SOUTHERN Co GAS | Operating Segments
Segment Reporting Information [Line Items]
Total assets	22,568		22,987
All Other | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	401	123
Segment net income	(74)	(84)
Total assets	2,733		2,552
SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Operating revenues	509	450
Segment net income	121	70
Total assets	15,182		15,206
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	1,639	1,560
Segment net income	279	239
Total assets	22,568		22,987
Goodwill impairment	42	0
SOUTHERN Co GAS | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	1,671	1,624
Segment net income	289	231
Total assets	23,642		24,842
SOUTHERN Co GAS | Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(33)	(66)
Segment net income	0	0
Total assets	(12,913)		(14,039)
SOUTHERN Co GAS | SOUTHERN Co GAS | Operating Segments
Segment Reporting Information [Line Items]
Segment net income	279
SOUTHERN Co GAS | All Other | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	1	2
Segment net income	(10)	8
Total assets	11,839		12,184
SOUTHERN Co GAS | Gas distribution operations | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	1,212	1,180
Segment net income	149	117
Total assets	18,332		19,358
SOUTHERN Co GAS | Gas marketing services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	271	288
Segment net income	13	31
Total assets	2,144		2,147
SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	166	131
Segment net income	104	68
Total assets	903		1,096
Less Gross Gas Costs	1,939	1,844
SOUTHERN Co GAS | Gas Midstream Operations | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	22	25
Segment net income	23	15
Total assets	2,263		2,241
Third Party Gross Revenues | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	1,938	1,839
Intercompany Revenues | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	167	136
Total Gross Revenues | SOUTHERN Co GAS | Wholesale Gas Services | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	2,105	1,975
Reportable Subsegments | Traditional Electric Operating Companies | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	3,979	3,786
Segment net income	612	432
Total assets	72,893		72,204
Reportable Subsegments | SOUTHERN POWER CO | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	509	450
Segment net income	121	70
Total assets	15,182		15,206
Intersubsegment Eliminations | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	(106)	(105)
Segment net income	0	$ 0
Total assets	$ (262)		$ (325)