SEGMENT AND RELATED INFORMATION - Financial Data for Business Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Operating revenues	$ 5,018	$ 5,412
Segment net income (loss)	868	2,084
Goodwill	5,280		$ 5,280
Total assets	118,852		118,700
SOUTHERN POWER CO
Segment Reporting Information [Line Items]
Operating revenues	375	443
Segment net income (loss)	75	56
Total assets	13,646		14,300
SOUTHERN POWER CO | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Plant Mankato
Segment Reporting Information [Line Items]
Gain on sale	39
Gain on sale, net of tax	23
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	1,249	1,474
Segment net income (loss)	275	270
Goodwill	5,015		5,015
Total assets	21,617		21,687
Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	3,695	3,795
Segment net income (loss)	717	621
Goodwill	2		2
Total assets	94,716		94,650
SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	1,249	1,474
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 Marketing Services | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Goodwill	981
Operating Segments | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	1,256	1,495
Segment net income (loss)	274	273
Total assets	21,978		22,228
Operating Segments | Traditional Electric Operating Companies | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	3,407	3,445
Segment net income (loss)	642	565
Goodwill	0		0
Total assets	81,765		81,063
Operating Segments | SOUTHERN POWER CO | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	375	443
Segment net income (loss)	75	56
Goodwill	2		2
Total assets	13,646		14,300
Operating Segments | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	1,249	1,474
Segment net income (loss)	275	270
Goodwill	5,015		5,015
Total assets	21,617		21,687
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	114	182
Segment net income (loss)	(121)	1,195
Goodwill	263		263
Total assets	3,467		3,511
Operating Segments | All Other | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	8	11
Segment net income (loss)	1	(3)
Total assets	11,094		10,759
Operating Segments | Gas Distribution Operations | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	1,020	1,172
Segment net income (loss)	164	133
Total assets	18,166		18,204
Operating Segments | Gas Pipeline Investments | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	8	8
Segment net income (loss)	30	32
Total assets	1,652		1,678
Operating Segments | Wholesale Gas Services | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	51	86
Segment net income (loss)	23	47
Total assets	643		850
Operating Segments | Gas Marketing Services | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	177	229
Segment net income (loss)	57	61
Total assets	1,517		1,496
Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(40)	(39)
Segment net income (loss)	(3)	(2)
Goodwill	0		0
Total assets	(948)		(1,148)
Eliminations | SOUTHERN Co GAS
Segment Reporting Information [Line Items]
Operating revenues	(15)	(32)
Segment net income (loss)	0	0
Total assets	(11,455)		(11,300)
Eliminations | Electric Utilities
Segment Reporting Information [Line Items]
Operating revenues	(87)	(93)
Segment net income (loss)	0	$ 0
Goodwill	0		0
Total assets	$ (695)		$ (713)