Segment and Related Information - Financial Data for Business Segments and Products and Services (Details)	3 Months Ended								6 Months Ended		10 Months Ended	12 Months Ended
	Dec. 31, 2018 USD ($) utility	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Oct. 31, 2016 USD ($)	Dec. 31, 2018 USD ($) utility	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Total operating revenues												$ 23,495,000,000	$ 23,031,000,000	$ 19,896,000,000
Depreciation and amortization												3,131,000,000	3,010,000,000	2,502,000,000
Interest income												38,000,000	26,000,000	20,000,000
Earnings from equity method investments											$ 15,000,000	148,000,000	106,000,000	59,000,000
Interest expense												1,842,000,000	1,694,000,000	1,317,000,000
Income taxes												449,000,000	142,000,000	951,000,000
Segment net income (loss)	$ 269,000,000	$ 1,222,000,000	$ (127,000,000)	$ 936,000,000	$ 500,000,000	$ 1,109,000,000	$ (1,348,000,000)	$ 665,000,000				2,226,000,000	842,000,000	2,448,000,000
Goodwill	5,315,000,000				6,268,000,000				$ 6,251,000,000			5,315,000,000	6,268,000,000	6,251,000,000
Total Assets	116,914,000,000				111,005,000,000				109,697,000,000			116,914,000,000	111,005,000,000	109,697,000,000
Gross property additions												8,205,000,000	5,984,000,000	7,624,000,000
Depreciation and amortization												3,549,000,000	3,457,000,000	2,923,000,000
Operating Income	578,000,000	2,174,000,000	63,000,000	1,376,000,000	739,000,000	1,991,000,000	(1,649,000,000)	1,252,000,000				4,191,000,000	2,333,000,000	4,486,000,000
Segment and Related Information (Textual) [Abstract]
Estimated loss on plants under construction												1,097,000,000	3,362,000,000	428,000,000
Impairment charges												210,000,000	0	0
Gain (loss) on asset sales												301,000,000	42,000,000	2,000,000
Goodwill impairment	0											42,000,000
Tax Cuts And Jobs Act Of 2017, income tax expense (benefit)					264,000,000
Kemper IGCC
Segment and Related Information (Textual) [Abstract]
After tax charge to income												722,000,000	2,400,000,000	264,000,000
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												18,571,000,000	18,540,000,000	17,941,000,000
Depreciation and amortization												2,565,000,000	2,457,000,000	2,233,000,000
Interest income												31,000,000	21,000,000	13,000,000
Earnings from equity method investments												(1,000,000)	1,000,000	2,000,000
Interest expense												1,035,000,000	1,011,000,000	931,000,000
Income taxes												207,000,000	82,000,000	1,091,000,000
Segment net income (loss)												2,304,000,000	878,000,000	2,571,000,000
Goodwill	2,000,000				2,000,000				2,000,000			2,000,000	2,000,000	2,000,000
Total Assets	93,959,000,000				87,085,000,000				86,994,000,000			93,959,000,000	87,085,000,000	86,994,000,000
Gross property additions												6,392,000,000	4,104,000,000	6,966,000,000
Traditional Operating Companies
Segment and Related Information (Textual) [Abstract]
Tax Cuts And Jobs Act Of 2017, income tax expense (benefit)													367,000,000
Traditional Operating Companies | Electric Utilities
Segment Reporting Information [Line Items]
Total operating revenues												16,843,000,000	16,884,000,000	16,803,000,000
Depreciation and amortization												2,072,000,000	1,954,000,000	1,881,000,000
Interest income												23,000,000	14,000,000	6,000,000
Earnings from equity method investments												(1,000,000)	1,000,000	2,000,000
Interest expense												852,000,000	820,000,000	814,000,000
Income taxes												371,000,000	1,021,000,000	1,286,000,000
Segment net income (loss)												2,117,000,000	(193,000,000)	2,233,000,000
Goodwill	0				0				0			0	0	0
Total Assets	79,382,000,000				72,204,000,000				72,141,000,000			79,382,000,000	72,204,000,000	72,141,000,000
Gross property additions												6,077,000,000	3,836,000,000	4,852,000,000
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												2,205,000,000	2,075,000,000	1,577,000,000
Depreciation and amortization												493,000,000	503,000,000	352,000,000
Interest income												8,000,000	7,000,000	7,000,000
Earnings from equity method investments												0	0	0
Interest expense												183,000,000	191,000,000	117,000,000
Income taxes												(164,000,000)	(939,000,000)	(195,000,000)
Segment net income (loss)												187,000,000	1,071,000,000	338,000,000
Goodwill	2,000,000				2,000,000				2,000,000			2,000,000	2,000,000	2,000,000
Total Assets	14,883,000,000				15,206,000,000				15,169,000,000			14,883,000,000	15,206,000,000	15,169,000,000
Gross property additions												315,000,000	268,000,000	2,114,000,000
Southern Company Gas
Segment Reporting Information [Line Items]
Total operating revenues												3,909,000,000	3,920,000,000	1,652,000,000
Depreciation and amortization												500,000,000	501,000,000	238,000,000
Interest income												4,000,000	3,000,000	2,000,000
Earnings from equity method investments												148,000,000	106,000,000	60,000,000
Interest expense												228,000,000	200,000,000	81,000,000
Income taxes												464,000,000	367,000,000	76,000,000
Segment net income (loss)												372,000,000	243,000,000	114,000,000
Goodwill	5,015,000,000				5,967,000,000				5,967,000,000			5,015,000,000	5,967,000,000	5,967,000,000
Total Assets	21,448,000,000				22,987,000,000				21,853,000,000			21,448,000,000	22,987,000,000	21,853,000,000
Gross property additions												1,399,000,000	1,525,000,000	618,000,000
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												1,213,000,000	741,000,000	463,000,000
Depreciation and amortization												66,000,000	52,000,000	31,000,000
Interest income												8,000,000	11,000,000	20,000,000
Earnings from equity method investments												2,000,000	(1,000,000)	(3,000,000)
Interest expense												580,000,000	490,000,000	317,000,000
Income taxes												(222,000,000)	(307,000,000)	(216,000,000)
Segment net income (loss)												(453,000,000)	(279,000,000)	(230,000,000)
Goodwill	298,000,000				299,000,000				282,000,000			298,000,000	299,000,000	282,000,000
Total Assets	3,285,000,000				2,552,000,000				2,474,000,000			3,285,000,000	2,552,000,000	2,474,000,000
Gross property additions												414,000,000	355,000,000	41,000,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Total operating revenues												(198,000,000)	(170,000,000)	(160,000,000)
Depreciation and amortization												0	0	0
Interest income												(5,000,000)	(9,000,000)	(15,000,000)
Earnings from equity method investments												(1,000,000)	0	0
Interest expense												(1,000,000)	(7,000,000)	(12,000,000)
Income taxes												0	0	0
Segment net income (loss)												3,000,000	0	(7,000,000)
Goodwill	0				0				0			0	0	0
Total Assets	(1,778,000,000)				(1,619,000,000)				(1,624,000,000)			(1,778,000,000)	(1,619,000,000)	(1,624,000,000)
Gross property additions												0	0	(1,000,000)
Intersegment Eliminations | Electric Utilities
Segment Reporting Information [Line Items]
Total operating revenues												(477,000,000)	(419,000,000)	(439,000,000)
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
Total Assets	(306,000,000)				(325,000,000)				(316,000,000)			(306,000,000)	(325,000,000)	(316,000,000)
Gross property additions												0	0	0
Southern Power
Segment Reporting Information [Line Items]
Total operating revenues												2,205,000,000	2,075,000,000	1,577,000,000
Depreciation and amortization												493,000,000	503,000,000	352,000,000
Interest expense												183,000,000	191,000,000	117,000,000
Income taxes	75,000,000											(164,000,000)	(939,000,000)	(195,000,000)
Segment net income (loss)	(60,000,000)	146,000,000	45,000,000	115,000,000	793,000,000	154,000,000	104,000,000	66,000,000				187,000,000	1,071,000,000	338,000,000
Total Assets	14,883,000,000				15,206,000,000							14,883,000,000	15,206,000,000
Depreciation and amortization												524,000,000	536,000,000	370,000,000
Operating Income	30,000,000	136,000,000	16,000,000	60,000,000	32,000,000	159,000,000	112,000,000	65,000,000				242,000,000	368,000,000	290,000,000
Segment and Related Information (Textual) [Abstract]
Impairment charges												156,000,000	0	0
After tax impairment charges												117,000,000
Gain (loss) on asset sales												(2,000,000)	0	0
Tax Cuts And Jobs Act Of 2017, income tax expense (benefit)					(743,000,000)
Southern Company Gas
Segment Reporting Information [Line Items]
Total operating revenues									1,652,000,000			3,909,000,000	3,920,000,000
Earnings from equity method investments									60,000,000			148,000,000	106,000,000
Interest expense									81,000,000			228,000,000	200,000,000
Income taxes									76,000,000			464,000,000	367,000,000
Segment net income (loss)	78,000,000	46,000,000	(31,000,000)	279,000,000	(60,000,000)	15,000,000	49,000,000	239,000,000	114,000,000			372,000,000	243,000,000
Goodwill	5,015,000,000				5,967,000,000							5,015,000,000	5,967,000,000
Total Assets	21,448,000,000				22,987,000,000				21,853,000,000			21,448,000,000	22,987,000,000	21,853,000,000
Gross property additions									632,000,000			1,521,000,000	1,508,000,000
Depreciation and amortization									238,000,000			500,000,000	501,000,000
Operating Income	104,000,000	$ 374,000,000	$ 49,000,000	388,000,000	109,000,000	$ 67,000,000	$ 95,000,000	$ 389,000,000	199,000,000			915,000,000	660,000,000
Segment and Related Information (Textual) [Abstract]
Gain (loss) on asset sales									0			291,000,000	0
Goodwill impairment	0			42,000,000					0			42,000,000	0
Tax Cuts And Jobs Act Of 2017, income tax expense (benefit)					93,000,000
Southern Company Gas | Gas Distribution Operations
Segment Reporting Information [Line Items]
Goodwill	4,034,000,000				4,702,000,000							4,034,000,000	4,702,000,000
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
Goodwill impairment												0
Southern Company Gas | Gas marketing services
Segment Reporting Information [Line Items]
Goodwill	981,000,000				1,265,000,000							981,000,000	1,265,000,000
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)
Goodwill impairment												42,000,000
Southern Company Gas | Intersegment Eliminations
Segment Reporting Information [Line Items]
Total operating revenues									(102,000,000)			(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	(11,582,000,000)				(14,039,000,000)				(14,167,000,000)			(11,582,000,000)	(14,039,000,000)	(14,167,000,000)
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									1,723,000,000			3,930,000,000	4,090,000,000
Earnings from equity method investments									58,000,000			145,000,000	103,000,000
Interest expense									119,000,000			227,000,000	191,000,000
Income taxes									76,000,000			495,000,000	311,000,000
Segment net income (loss)									125,000,000			435,000,000	358,000,000
Total Assets	21,918,000,000				24,300,000,000				24,323,000,000			21,918,000,000	24,300,000,000	24,323,000,000
Gross property additions									618,000,000			1,467,000,000	1,457,000,000
Depreciation and amortization									221,000,000			453,000,000	457,000,000
Operating Income									251,000,000			1,013,000,000	717,000,000
Southern Company Gas | Operating Segments | All other
Segment Reporting Information [Line Items]
Total operating revenues									31,000,000			55,000,000	64,000,000
Earnings from equity method investments									2,000,000			3,000,000	3,000,000
Interest expense									(38,000,000)			1,000,000	9,000,000
Income taxes									0			(31,000,000)	56,000,000
Segment net income (loss)									(11,000,000)			(63,000,000)	(115,000,000)
Total Assets	11,112,000,000				12,726,000,000				11,697,000,000			11,112,000,000	12,726,000,000	11,697,000,000
Gross property additions									14,000,000			54,000,000	51,000,000
Depreciation and amortization									17,000,000			47,000,000	44,000,000
Operating Income									(52,000,000)			(98,000,000)	(57,000,000)
Southern Company Gas | Operating Segments | Gas Distribution Operations
Segment Reporting Information [Line Items]
Total operating revenues									1,342,000,000			3,186,000,000	3,207,000,000
Earnings from equity method investments									0			0	0
Interest expense									105,000,000			178,000,000	153,000,000
Income taxes									51,000,000			409,000,000	178,000,000
Segment net income (loss)									77,000,000			334,000,000	353,000,000
Total Assets	17,266,000,000				19,358,000,000				19,453,000,000			17,266,000,000	19,358,000,000	19,453,000,000
Gross property additions									561,000,000			1,429,000,000	1,330,000,000
Depreciation and amortization									185,000,000			409,000,000	391,000,000
Operating Income									225,000,000			904,000,000	645,000,000
Segment and Related Information (Textual) [Abstract]
Revenue on disposition									168,000,000			244,000,000	399,000,000
Southern Company Gas | Operating Segments | Gas Pipeline Investments
Segment Reporting Information [Line Items]
Total operating revenues									3,000,000			32,000,000	17,000,000
Earnings from equity method investments									58,000,000			145,000,000	103,000,000
Interest expense									10,000,000			34,000,000	26,000,000
Income taxes									21,000,000			28,000,000	109,000,000
Segment net income (loss)									29,000,000			103,000,000	(22,000,000)
Total Assets	1,763,000,000				1,699,000,000				1,659,000,000			1,763,000,000	1,699,000,000	1,659,000,000
Gross property additions									51,000,000			32,000,000	117,000,000
Depreciation and amortization									0			5,000,000	2,000,000
Operating Income									1,000,000			20,000,000	10,000,000
Southern Company Gas | Operating Segments | Wholesale gas services
Segment Reporting Information [Line Items]
Total operating revenues									24,000,000			144,000,000	6,000,000
Earnings from equity method investments									0			0	0
Interest expense									3,000,000			9,000,000	7,000,000
Income taxes									(3,000,000)			4,000,000	0
Segment net income (loss)									0			38,000,000	(57,000,000)
Total Assets	1,302,000,000				1,096,000,000				1,127,000,000			1,302,000,000	1,096,000,000	1,127,000,000
Gross property additions									1,000,000			0	1,000,000
Less Gross Gas Costs									6,116,000,000			7,262,000,000	6,627,000,000
Depreciation and amortization									1,000,000			2,000,000	2,000,000
Operating Income									(2,000,000)			70,000,000	(51,000,000)
Southern Company Gas | Operating Segments | Gas marketing services
Segment Reporting Information [Line Items]
Total operating revenues									354,000,000			568,000,000	860,000,000
Earnings from equity method investments									0			0	0
Interest expense									1,000,000			6,000,000	5,000,000
Income taxes									7,000,000			54,000,000	24,000,000
Segment net income (loss)									19,000,000			(40,000,000)	84,000,000
Total Assets	$ 1,587,000,000				$ 2,147,000,000				2,084,000,000			1,587,000,000	2,147,000,000	$ 2,084,000,000
Gross property additions									5,000,000			6,000,000	9,000,000
Depreciation and amortization									35,000,000			37,000,000	62,000,000
Operating Income									27,000,000			19,000,000	113,000,000
Segment and Related Information (Textual) [Abstract]
Revenue on disposition									56,000,000			55,000,000	129,000,000
Predecessor | Southern Company Gas
Segment Reporting Information [Line Items]
Total operating revenues										$ 1,905,000,000
Earnings from equity method investments										2,000,000
Interest expense										96,000,000
Income taxes										87,000,000
Segment net income (loss)										131,000,000
Gross property additions										548,000,000
Depreciation and amortization										206,000,000
Operating Income										323,000,000
EBIT										328,000,000
Segment and Related Information (Textual) [Abstract]
Gain (loss) on asset sales										0
Goodwill impairment										0
Predecessor | Southern Company Gas | Intersegment Eliminations
Segment Reporting Information [Line Items]
Total operating revenues										(102,000,000)
Gross property additions										0
Depreciation and amortization										0
Operating Income										0
EBIT										0
Predecessor | Southern Company Gas | Operating Segments
Segment Reporting Information [Line Items]
Total operating revenues										1,981,000,000
Gross property additions										529,000,000
Depreciation and amortization										190,000,000
Operating Income										396,000,000
EBIT										397,000,000
Predecessor | Southern Company Gas | Operating Segments | All other
Segment Reporting Information [Line Items]
Total operating revenues										26,000,000
Gross property additions										19,000,000
Depreciation and amortization										16,000,000
Operating Income										(73,000,000)
EBIT										(69,000,000)
Predecessor | Southern Company Gas | Operating Segments | Gas Distribution Operations
Segment Reporting Information [Line Items]
Total operating revenues										1,575,000,000
Gross property additions										484,000,000
Depreciation and amortization										178,000,000
Operating Income										353,000,000
EBIT										353,000,000
Segment and Related Information (Textual) [Abstract]
Revenue on disposition										215,000,000
Predecessor | Southern Company Gas | Operating Segments | Gas Pipeline Investments
Segment Reporting Information [Line Items]
Total operating revenues										3,000,000
Gross property additions										40,000,000
Depreciation and amortization										0
Operating Income										3,000,000
EBIT										3,000,000
Predecessor | Southern Company Gas | Operating Segments | Wholesale gas services
Segment Reporting Information [Line Items]
Total operating revenues										(32,000,000)
Gross property additions										1,000,000
Less Gross Gas Costs										2,675,000,000
Depreciation and amortization										1,000,000
Operating Income										(69,000,000)
EBIT										(68,000,000)
Predecessor | Southern Company Gas | Operating Segments | Gas marketing services
Segment Reporting Information [Line Items]
Total operating revenues										435,000,000
Gross property additions										4,000,000
Depreciation and amortization										11,000,000
Operating Income										109,000,000
EBIT										109,000,000
Segment and Related Information (Textual) [Abstract]
Revenue on disposition										64,000,000
Third Party Gross Revenues | Southern Company Gas | Operating Segments | Wholesale gas services
Segment Reporting Information [Line Items]
Total operating revenues									5,807,000,000			6,955,000,000	6,152,000,000
Third Party Gross Revenues | Predecessor | Southern Company Gas | Operating Segments | Wholesale gas services
Segment Reporting Information [Line Items]
Total operating revenues										2,500,000,000
Intercompany Revenues | Southern Company Gas | Operating Segments | Wholesale gas services
Segment Reporting Information [Line Items]
Total operating revenues									333,000,000			451,000,000	481,000,000
Intercompany Revenues | Predecessor | Southern Company Gas | Operating Segments | Wholesale gas services
Segment Reporting Information [Line Items]
Total operating revenues										143,000,000
Total Gross Revenues | Southern Company Gas | Operating Segments | Wholesale gas services
Segment Reporting Information [Line Items]
Total operating revenues									$ 6,140,000,000			$ 7,406,000,000	$ 6,633,000,000
Total Gross Revenues | Predecessor | Southern Company Gas | Operating Segments | Wholesale gas services
Segment Reporting Information [Line Items]
Total operating revenues										$ 2,643,000,000
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Southern Company Gas
Segment and Related Information (Textual) [Abstract]
Number of natural gas distribution utilities disposed | utility	3											3
Pivotal Home Solutions | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Southern Company Gas
Segment and Related Information (Textual) [Abstract]
Gain (loss) on asset sales												$ 291,000,000
Net gain (loss) on dispositions, net of tax												(51,000,000)
Goodwill impairment				$ 42,000,000								$ 42,000,000