SEGMENT AND RELATED INFORMATION - Financial Data for Business Segments (Details)		1 Months Ended	3 Months Ended									12 Months Ended
	Jan. 01, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2019 USD ($) utility	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) utility	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Total operating revenues												$ 21,419,000,000	$ 23,495,000,000	$ 23,031,000,000
Depreciation and amortization												3,038,000,000	3,131,000,000	3,010,000,000
Interest income												60,000,000	38,000,000	26,000,000
Earnings from equity method investments												162,000,000	148,000,000	106,000,000
Interest expense												1,736,000,000	1,842,000,000	1,694,000,000
Income taxes												1,798,000,000	449,000,000	142,000,000
Segment net income (loss)			$ 409,000,000	$ 1,345,000,000	$ 931,000,000	$ 2,059,000,000	$ 269,000,000	$ 1,222,000,000	$ (127,000,000)	$ 936,000,000		4,739,000,000	2,226,000,000	842,000,000
Goodwill			5,280,000,000				5,315,000,000					5,280,000,000	5,315,000,000	6,268,000,000
Total Assets			118,700,000,000				116,914,000,000					118,700,000,000	116,914,000,000	111,005,000,000
Gross property additions												7,814,000,000	8,205,000,000	5,984,000,000
Depreciation and amortization												3,331,000,000	3,549,000,000	3,457,000,000
Operating Income			690,000,000	2,013,000,000	1,342,000,000	3,691,000,000	578,000,000	2,174,000,000	63,000,000	1,376,000,000		7,736,000,000	4,191,000,000	2,333,000,000
Segment and Related Information (Textual) [Abstract]
Estimated loss on plants under construction												24,000,000	1,097,000,000	3,362,000,000
Gain (loss) on dispositions												(2,569,000,000)	(291,000,000)	(40,000,000)
Impairment charges												168,000,000	210,000,000	0
Lease, impairment loss			17,000,000
Lease, impairment loss, after tax			13,000,000
Gain (loss) on asset sales												2,588,000,000	301,000,000	42,000,000
Impairment charges			0
Plant Scherer Unit 3
Segment and Related Information (Textual) [Abstract]
After tax charge to income											$ 20,000,000
Pre-tax charge to income											$ 33,000,000
Electric Utilities
Segment Reporting Information [Line Items]
Total operating revenues												17,095,000,000	18,571,000,000	18,540,000,000
Depreciation and amortization												2,472,000,000	2,565,000,000	2,457,000,000
Interest income												47,000,000	31,000,000	21,000,000
Earnings from equity method investments												5,000,000	(1,000,000)	1,000,000
Interest expense												987,000,000	1,035,000,000	1,011,000,000
Income taxes												708,000,000	207,000,000	82,000,000
Segment net income (loss)												3,268,000,000	2,304,000,000	878,000,000
Goodwill			2,000,000				2,000,000					2,000,000	2,000,000	2,000,000
Total Assets			94,650,000,000				93,959,000,000					94,650,000,000	93,959,000,000	87,085,000,000
Gross property additions												6,237,000,000	6,392,000,000	4,104,000,000
Traditional Operating Companies
Segment and Related Information (Textual) [Abstract]
Pre-tax charge to income												24,000,000	1,100,000,000	3,400,000,000
Tax Cuts And Jobs Act Of 2017, income tax expense (benefit)													367,000,000
Traditional Operating Companies | Kemper IGCC
Segment and Related Information (Textual) [Abstract]
After tax charge to income												24,000,000	722,000,000	2,400,000,000
Traditional Operating Companies | Electric Utilities
Segment Reporting Information [Line Items]
Total operating revenues												15,569,000,000	16,843,000,000	16,884,000,000
Depreciation and amortization												1,993,000,000	2,072,000,000	1,954,000,000
Interest income												38,000,000	23,000,000	14,000,000
Earnings from equity method investments												2,000,000	(1,000,000)	1,000,000
Interest expense												818,000,000	852,000,000	820,000,000
Income taxes												764,000,000	371,000,000	1,021,000,000
Segment net income (loss)												2,929,000,000	2,117,000,000	(193,000,000)
Goodwill			0				0					0	0	0
Total Assets			81,063,000,000				79,382,000,000					81,063,000,000	79,382,000,000	72,204,000,000
Gross property additions												5,748,000,000	6,077,000,000	3,836,000,000
Southern Power
Segment and Related Information (Textual) [Abstract]
Tax Cuts And Jobs Act Of 2017, income tax expense (benefit)													743,000,000
Southern Power | Electric Utilities
Segment Reporting Information [Line Items]
Total operating revenues												1,938,000,000	2,205,000,000	2,075,000,000
Depreciation and amortization												479,000,000	493,000,000	503,000,000
Interest income												9,000,000	8,000,000	7,000,000
Earnings from equity method investments												3,000,000	0	0
Interest expense												169,000,000	183,000,000	191,000,000
Income taxes												(56,000,000)	(164,000,000)	(939,000,000)
Segment net income (loss)												339,000,000	187,000,000	1,071,000,000
Goodwill			2,000,000				2,000,000					2,000,000	2,000,000	2,000,000
Total Assets			14,300,000,000				14,883,000,000					14,300,000,000	14,883,000,000	15,206,000,000
Gross property additions												489,000,000	315,000,000	268,000,000
Southern Company Gas
Segment Reporting Information [Line Items]
Total operating revenues												3,792,000,000	3,909,000,000	3,920,000,000
Depreciation and amortization												487,000,000	500,000,000	501,000,000
Interest income												3,000,000	4,000,000	3,000,000
Earnings from equity method investments												157,000,000	148,000,000	106,000,000
Interest expense												232,000,000	228,000,000	200,000,000
Income taxes												130,000,000	464,000,000	367,000,000
Segment net income (loss)												585,000,000	372,000,000	243,000,000
Goodwill			5,015,000,000				5,015,000,000					5,015,000,000	5,015,000,000	5,967,000,000
Total Assets			21,687,000,000				21,448,000,000					21,687,000,000	21,448,000,000	22,987,000,000
Gross property additions												1,418,000,000	1,399,000,000	1,525,000,000
Segment and Related Information (Textual) [Abstract]
Tax Cuts And Jobs Act Of 2017, income tax expense (benefit)													93,000,000
All other
Segment Reporting Information [Line Items]
Total operating revenues												690,000,000	1,213,000,000	741,000,000
Depreciation and amortization												79,000,000	66,000,000	52,000,000
Interest income												16,000,000	8,000,000	11,000,000
Earnings from equity method investments												0	2,000,000	(1,000,000)
Interest expense												517,000,000	580,000,000	490,000,000
Income taxes												960,000,000	(222,000,000)	(307,000,000)
Segment net income (loss)												908,000,000	(453,000,000)	(279,000,000)
Goodwill			263,000,000				298,000,000					263,000,000	298,000,000	299,000,000
Total Assets			3,511,000,000				3,285,000,000					3,511,000,000	3,285,000,000	2,552,000,000
Gross property additions												159,000,000	414,000,000	355,000,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Total operating revenues												(158,000,000)	(198,000,000)	(170,000,000)
Depreciation and amortization												0	0	0
Interest income												(6,000,000)	(5,000,000)	(9,000,000)
Earnings from equity method investments												0	(1,000,000)	0
Interest expense												0	(1,000,000)	(7,000,000)
Income taxes												0	0	0
Segment net income (loss)												(22,000,000)	3,000,000	0
Goodwill			0				0					0	0	0
Total Assets			(1,148,000,000)				(1,778,000,000)					(1,148,000,000)	(1,778,000,000)	(1,619,000,000)
Gross property additions												0	0	0
Intersegment Eliminations | Electric Utilities
Segment Reporting Information [Line Items]
Total operating revenues												(412,000,000)	(477,000,000)	(419,000,000)
Depreciation and amortization												0	0	0
Interest income												0	0	0
Earnings from equity method investments												0	0	0
Interest expense												0	0	0
Income taxes												0	0	0
Segment net income (loss)												0	0	0
Goodwill			0				0					0	0	0
Total Assets			(713,000,000)				(306,000,000)					(713,000,000)	(306,000,000)	(325,000,000)
Gross property additions												0	0	0
Southern Power
Segment Reporting Information [Line Items]
Total operating revenues												1,938,000,000	2,205,000,000	2,075,000,000
Depreciation and amortization												479,000,000	493,000,000	503,000,000
Interest expense												169,000,000	183,000,000	191,000,000
Income taxes							75,000,000					(56,000,000)	(164,000,000)	(939,000,000)
Segment net income (loss)			(12,000,000)	111,000,000	203,000,000	27,000,000	(60,000,000)	146,000,000	45,000,000	115,000,000		339,000,000	187,000,000	1,071,000,000
Total Assets			14,300,000,000				14,883,000,000					14,300,000,000	14,883,000,000
Depreciation and amortization												505,000,000	524,000,000	536,000,000
Operating Income			15,000,000	167,000,000	153,000,000	60,000,000	30,000,000	136,000,000	16,000,000	60,000,000		395,000,000	242,000,000	368,000,000
Segment and Related Information (Textual) [Abstract]
Impairment charges												3,000,000	156,000,000	0
After tax impairment charges													117,000,000
Gain (loss) on asset sales												(23,000,000)	(2,000,000)	0
Southern Company Gas
Segment Reporting Information [Line Items]
Total operating revenues												3,792,000,000	3,909,000,000	3,920,000,000
Earnings from equity method investments												157,000,000	148,000,000	106,000,000
Interest expense												232,000,000	228,000,000	200,000,000
Income taxes												130,000,000	464,000,000	367,000,000
Segment net income (loss)			238,000,000	(29,000,000)	106,000,000	270,000,000	78,000,000	46,000,000	(31,000,000)	279,000,000		585,000,000	372,000,000	243,000,000
Goodwill			5,015,000,000				5,015,000,000					5,015,000,000	5,015,000,000
Total Assets			21,687,000,000				21,448,000,000					21,687,000,000	21,448,000,000	22,987,000,000
Gross property additions												1,466,000,000	1,521,000,000	1,508,000,000
Depreciation and amortization												487,000,000	500,000,000	501,000,000
Operating Income			318,000,000	(35,000,000)	134,000,000	353,000,000	104,000,000	374,000,000	49,000,000	388,000,000		770,000,000	915,000,000	660,000,000
Segment and Related Information (Textual) [Abstract]
Gain (loss) on dispositions												0	(291,000,000)	0
Gain (loss) on asset sales												0	291,000,000	0
Impairment charges			0									115,000,000	42,000,000	0
Southern Company Gas | Gas Distribution Operations
Segment Reporting Information [Line Items]
Goodwill			4,034,000,000				4,034,000,000					4,034,000,000	4,034,000,000
Segment and Related Information (Textual) [Abstract]
Gain (loss) on asset sales												324,000,000
Net gain (loss) on dispositions, net of tax												16,000,000
Southern Company Gas | Gas marketing services
Segment Reporting Information [Line Items]
Goodwill			981,000,000				981,000,000					981,000,000	981,000,000
Segment and Related Information (Textual) [Abstract]
Gain (loss) on asset sales												(33,000,000)
Net gain (loss) on dispositions, net of tax												(67,000,000)
Southern Company Gas | Intersegment Eliminations
Segment Reporting Information [Line Items]
Total operating revenues												(62,000,000)	(76,000,000)	(234,000,000)
Earnings from equity method investments												0	0	0
Interest expense												0	0	0
Income taxes												0	0	0
Segment net income (loss)												0	0	0
Total Assets			(11,300,000,000)				(11,582,000,000)					(11,300,000,000)	(11,582,000,000)	(14,039,000,000)
Gross property additions												0	0	0
Depreciation and amortization												0	0	0
Operating Income												0	0	0
Southern Company Gas | Operating Segments
Segment Reporting Information [Line Items]
Total operating revenues												3,810,000,000	3,930,000,000	4,090,000,000
Earnings from equity method investments												162,000,000	145,000,000	103,000,000
Interest expense												225,000,000	227,000,000	191,000,000
Income taxes												200,000,000	495,000,000	311,000,000
Segment net income (loss)												677,000,000	435,000,000	358,000,000
Total Assets			22,228,000,000				21,918,000,000					22,228,000,000	21,918,000,000	24,300,000,000
Gross property additions												1,439,000,000	1,467,000,000	1,457,000,000
Depreciation and amortization												454,000,000	453,000,000	457,000,000
Operating Income												924,000,000	1,013,000,000	717,000,000
Southern Company Gas | Operating Segments | All other
Segment Reporting Information [Line Items]
Total operating revenues												44,000,000	55,000,000	64,000,000
Earnings from equity method investments												(5,000,000)	3,000,000	3,000,000
Interest expense												7,000,000	1,000,000	9,000,000
Income taxes												(70,000,000)	(31,000,000)	56,000,000
Segment net income (loss)												(92,000,000)	(63,000,000)	(115,000,000)
Total Assets			10,759,000,000				11,112,000,000					10,759,000,000	11,112,000,000	12,726,000,000
Gross property additions												27,000,000	54,000,000	51,000,000
Depreciation and amortization												33,000,000	47,000,000	44,000,000
Operating Income												(154,000,000)	(98,000,000)	(57,000,000)
Southern Company Gas | Operating Segments | Gas Distribution Operations
Segment Reporting Information [Line Items]
Total operating revenues												3,028,000,000	3,186,000,000	3,207,000,000
Earnings from equity method investments												0	0	0
Interest expense												187,000,000	178,000,000	153,000,000
Income taxes												63,000,000	409,000,000	178,000,000
Segment net income (loss)												337,000,000	334,000,000	353,000,000
Total Assets			18,204,000,000				17,266,000,000					18,204,000,000	17,266,000,000	19,358,000,000
Gross property additions												1,433,000,000	1,429,000,000	1,330,000,000
Depreciation and amortization												422,000,000	409,000,000	391,000,000
Operating Income												573,000,000	904,000,000	645,000,000
Segment and Related Information (Textual) [Abstract]
Disposal group, including discontinued operation, revenue												244,000,000	399,000,000
Southern Company Gas | Operating Segments | Gas Pipeline Investments
Segment Reporting Information [Line Items]
Total operating revenues												32,000,000	32,000,000	17,000,000
Earnings from equity method investments												162,000,000	145,000,000	103,000,000
Interest expense												30,000,000	34,000,000	26,000,000
Income taxes												58,000,000	28,000,000	109,000,000
Segment net income (loss)												94,000,000	103,000,000	(22,000,000)
Total Assets			1,678,000,000				1,763,000,000					1,678,000,000	1,763,000,000	1,699,000,000
Gross property additions												1,000,000	32,000,000	117,000,000
Depreciation and amortization												5,000,000	5,000,000	2,000,000
Operating Income												20,000,000	20,000,000	10,000,000
Southern Company Gas | Operating Segments | Wholesale gas services
Segment Reporting Information [Line Items]
Total operating revenues												294,000,000	144,000,000	6,000,000
Earnings from equity method investments												0	0	0
Interest expense												5,000,000	9,000,000	7,000,000
Income taxes												52,000,000	4,000,000	0
Segment net income (loss)												163,000,000	38,000,000	(57,000,000)
Total Assets			850,000,000				1,302,000,000					850,000,000	1,302,000,000	1,096,000,000
Gross property additions												1,000,000	0	1,000,000
Less Gross Gas Costs												5,684,000,000	7,262,000,000	6,627,000,000
Depreciation and amortization												1,000,000	2,000,000	2,000,000
Operating Income												219,000,000	70,000,000	(51,000,000)
Southern Company Gas | Operating Segments | Gas marketing services
Segment Reporting Information [Line Items]
Total operating revenues												456,000,000	568,000,000	860,000,000
Earnings from equity method investments												0	0	0
Interest expense												3,000,000	6,000,000	5,000,000
Income taxes												27,000,000	54,000,000	24,000,000
Segment net income (loss)												83,000,000	(40,000,000)	84,000,000
Total Assets			$ 1,496,000,000				1,587,000,000					1,496,000,000	1,587,000,000	2,147,000,000
Gross property additions												4,000,000	6,000,000	9,000,000
Depreciation and amortization												26,000,000	37,000,000	62,000,000
Operating Income												112,000,000	19,000,000	113,000,000
Segment and Related Information (Textual) [Abstract]
Disposal group, including discontinued operation, revenue												55,000,000	129,000,000
Third Party Gross Revenues | Southern Company Gas | Operating Segments | Wholesale gas services
Segment Reporting Information [Line Items]
Total operating revenues												5,703,000,000	6,955,000,000	6,152,000,000
Intercompany Revenues | Southern Company Gas | Operating Segments | Wholesale gas services
Segment Reporting Information [Line Items]
Total operating revenues												275,000,000	451,000,000	481,000,000
Total Gross Revenues | Southern Company Gas | Operating Segments | Wholesale gas services
Segment Reporting Information [Line Items]
Total operating revenues												$ 5,978,000,000	$ 7,406,000,000	$ 6,633,000,000
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Southern Company Gas
Segment and Related Information (Textual) [Abstract]
Number of distribution utilities | utility			3,000,000,000									3,000,000,000
Gain (loss) on dispositions							(27,000,000)	353,000,000	(36,000,000)
Gain (loss) on dispositions, after tax							$ (15,000,000)	$ 40,000,000	$ (76,000,000)
Plant Nacogdoches | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Southern Power
Segment and Related Information (Textual) [Abstract]
Gain (loss) on dispositions					23,000,000							$ 23,000,000
Gain (loss) on dispositions, after tax					88,000,000							88,000,000
Gulf Power | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Segment and Related Information (Textual) [Abstract]
Gain (loss) on dispositions	$ 2,600,000,000		$ 70,000,000	4,000,000	(15,000,000)	2,500,000,000						2,600,000,000
Gain (loss) on dispositions, after tax	$ 1,400,000,000		102,000,000	4,000,000	(11,000,000)	$ 1,300,000,000						1,400,000,000
PowerSecure International, Inc. Lighting Business | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Segment and Related Information (Textual) [Abstract]
Gain (loss) on dispositions												(58,000,000)
Gain (loss) on dispositions, after tax												(52,000,000)
Impairment charges			12,000,000	14,000,000	$ 32,000,000
After tax impairment charges			$ 5,000,000	$ 15,000,000
Pivotal Home Solutions | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Southern Company Gas
Segment and Related Information (Textual) [Abstract]
Gain (loss) on dispositions												(24,000,000)
Gain (loss) on dispositions, after tax		$ (67,000,000)										(17,000,000)
Gain (loss) on asset sales												291,000,000
Net gain (loss) on dispositions, net of tax												(51,000,000)
Impairment charges										$ 42,000,000		42,000,000
Natural Gas Storage Facility | Southern Company Gas
Segment and Related Information (Textual) [Abstract]
Impairment charges												115,000,000
After tax impairment charges												$ 86,000,000