Segment and Related Information - Financial Data for Business Segments and Products and Services (Details) - USD ($)	3 Months Ended								6 Months Ended		10 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2016	Jun. 30, 2016	Oct. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financial data for business segments
Operating revenues	$ 5,629,000,000	$ 6,201,000,000	$ 5,430,000,000	$ 5,771,000,000	$ 5,181,000,000	$ 6,264,000,000	$ 4,459,000,000	$ 3,992,000,000				$ 23,031,000,000	$ 19,896,000,000	$ 17,489,000,000
Depreciation and amortization												3,010,000,000	2,502,000,000	2,034,000,000
Depreciation and amortization												3,457,000,000	2,923,000,000	2,395,000,000
Operating Income (Loss)	794,000,000	2,045,000,000	(1,594,000,000)	1,306,000,000	587,000,000	1,917,000,000	1,185,000,000	940,000,000				2,551,000,000	4,629,000,000	4,282,000,000
Earnings from equity method investments											$ 15,000,000	106,000,000	59,000,000	0
Interest income												26,000,000	20,000,000	23,000,000
Interest expense												1,694,000,000	1,317,000,000	840,000,000
Income taxes												142,000,000	951,000,000	1,194,000,000
Segment net income (loss)	496,000,000	1,069,000,000	(1,381,000,000)	658,000,000	197,000,000	1,139,000,000	623,000,000	489,000,000				842,000,000	2,448,000,000	2,367,000,000
Gross property additions												5,984,000,000	7,624,000,000	6,169,000,000
Total assets	111,005,000,000				109,697,000,000				$ 109,697,000,000			111,005,000,000	109,697,000,000	78,318,000,000
Segment and Related Information (Textual) [Abstract]
Estimated loss on Kemper IGCC												3,362,000,000	428,000,000	365,000,000
Unamortized debt issuance expense	226,000,000				213,000,000				213,000,000			226,000,000	213,000,000
Kemper IGCC
Segment and Related Information (Textual) [Abstract]
After tax charge to income	185,000,000	21,000,000	2,100,000,000	67,000,000	127,000,000	54,000,000	50,000,000	33,000,000				2,400,000,000	264,000,000	226,000,000
Electric Utilities
Financial data for business segments
Operating revenues												18,540,000,000	17,941,000,000	17,442,000,000
Depreciation and amortization												2,457,000,000	2,233,000,000	2,020,000,000
Earnings from equity method investments												1,000,000	2,000,000	1,000,000
Interest income												21,000,000	13,000,000	22,000,000
Interest expense												1,011,000,000	931,000,000	774,000,000
Income taxes												82,000,000	1,091,000,000	1,326,000,000
Segment net income (loss)												878,000,000	2,571,000,000	2,401,000,000
Gross property additions												4,104,000,000	6,966,000,000	6,129,000,000
Total assets	87,085,000,000				86,994,000,000				86,994,000,000			87,085,000,000	86,994,000,000	77,560,000,000
SOUTHERN POWER CO
Financial data for business segments
Operating revenues												392,000,000	419,000,000	417,000,000
SOUTHERN POWER CO | Electric Utilities
Financial data for business segments
Operating revenues												2,075,000,000	1,577,000,000	1,390,000,000
Depreciation and amortization												503,000,000	352,000,000	248,000,000
Earnings from equity method investments												0	0	0
Interest income												7,000,000	7,000,000	2,000,000
Interest expense												191,000,000	117,000,000	77,000,000
Income taxes												(939,000,000)	(195,000,000)	21,000,000
Segment net income (loss)												1,071,000,000	338,000,000	215,000,000
Gross property additions												268,000,000	2,114,000,000	1,005,000,000
Total assets	15,206,000,000				15,169,000,000				15,169,000,000			15,206,000,000	15,169,000,000	8,905,000,000
SOUTHERN Co GAS | Electric Utilities
Financial data for business segments
Operating revenues												3,920,000,000	1,652,000,000	0
Depreciation and amortization												501,000,000	238,000,000	0
Earnings from equity method investments												106,000,000	60,000,000	0
Interest income												3,000,000	2,000,000	0
Interest expense												200,000,000	81,000,000	0
Income taxes												367,000,000	76,000,000	0
Segment net income (loss)												243,000,000	114,000,000	0
Gross property additions												1,525,000,000	618,000,000	0
Total assets	22,987,000,000				21,853,000,000				21,853,000,000			22,987,000,000	21,853,000,000	0
Traditional Operating Companies | Electric Utilities
Financial data for business segments
Operating revenues												16,884,000,000	16,803,000,000	16,491,000,000
Depreciation and amortization												1,954,000,000	1,881,000,000	1,772,000,000
Earnings from equity method investments												1,000,000	2,000,000	1,000,000
Interest income												14,000,000	6,000,000	19,000,000
Interest expense												820,000,000	814,000,000	697,000,000
Income taxes												1,021,000,000	1,286,000,000	1,305,000,000
Segment net income (loss)												(193,000,000)	2,233,000,000	2,186,000,000
Gross property additions												3,836,000,000	4,852,000,000	5,124,000,000
Total assets	72,204,000,000				72,141,000,000				72,141,000,000			72,204,000,000	72,141,000,000	69,052,000,000
All Other [Member]
Financial data for business segments
Operating revenues												741,000,000	463,000,000	152,000,000
Depreciation and amortization												52,000,000	31,000,000	14,000,000
Earnings from equity method investments												(1,000,000)	(3,000,000)	(1,000,000)
Interest income												11,000,000	20,000,000	6,000,000
Interest expense												490,000,000	317,000,000	69,000,000
Income taxes												(307,000,000)	(216,000,000)	(132,000,000)
Segment net income (loss)												(279,000,000)	(230,000,000)	(32,000,000)
Gross property additions												355,000,000	41,000,000	40,000,000
Total assets	2,552,000,000				2,474,000,000				2,474,000,000			2,552,000,000	2,474,000,000	1,819,000,000
Intersegment Eliminations
Financial data for business segments
Operating revenues												(170,000,000)	(160,000,000)	(105,000,000)
Depreciation and amortization												0	0	0
Earnings from equity method investments												0	0	0
Interest income												(9,000,000)	(15,000,000)	(5,000,000)
Interest expense												(7,000,000)	(12,000,000)	(3,000,000)
Income taxes												0	0	0
Segment net income (loss)												0	(7,000,000)	(2,000,000)
Gross property additions												0	(1,000,000)	0
Total assets	(1,619,000,000)				(1,624,000,000)				(1,624,000,000)			(1,619,000,000)	(1,624,000,000)	(1,061,000,000)
Intersegment Eliminations | Electric Utilities
Financial data for business segments
Operating revenues												(419,000,000)	(439,000,000)	(439,000,000)
Depreciation and amortization												0	0	0
Earnings from equity method investments												0	0	0
Interest income												0	0	1,000,000
Interest expense												0	0	0
Income taxes												0	0	0
Segment net income (loss)												0	0	0
Gross property additions												0	0	0
Total assets	(325,000,000)				(316,000,000)				(316,000,000)			(325,000,000)	(316,000,000)	(397,000,000)
GULF POWER CO
Financial data for business segments
Operating revenues	372,000,000	437,000,000	357,000,000	350,000,000	349,000,000	436,000,000	365,000,000	335,000,000				1,516,000,000	1,485,000,000	1,483,000,000
Depreciation and amortization												137,000,000	172,000,000	141,000,000
Depreciation and amortization												149,000,000	179,000,000	152,000,000
Operating Income (Loss)	53,000,000	115,000,000	75,000,000	46,000,000	54,000,000	90,000,000	74,000,000	65,000,000				289,000,000	283,000,000	290,000,000
Interest expense												50,000,000	47,000,000	49,000,000
Income taxes												90,000,000	91,000,000	92,000,000
Segment net income (loss)	19,000,000	63,000,000	35,000,000	18,000,000	23,000,000	45,000,000	34,000,000	29,000,000				135,000,000	131,000,000	148,000,000
Total assets	4,797,000,000				4,822,000,000				4,822,000,000			4,797,000,000	4,822,000,000
Segment and Related Information (Textual) [Abstract]
Loss on Plant Scherer Unit 3												33,000,000	0	0
After tax charge to income												0
Unamortized debt issuance expense	9,000,000				7,000,000				7,000,000			9,000,000	7,000,000
SOUTHERN Co GAS
Financial data for business segments
Operating revenues	1,079,000,000	565,000,000	716,000,000	1,560,000,000	1,109,000,000	543,000,000			1,652,000,000			3,920,000,000
Depreciation and amortization									238,000,000			501,000,000
Operating Income (Loss)	110,000,000	68,000,000	96,000,000	391,000,000	185,000,000	12,000,000			197,000,000			665,000,000
Earnings from equity method investments									60,000,000			106,000,000
EBIT	129,000,000	118,000,000	128,000,000	435,000,000	221,000,000	50,000,000
Interest expense									81,000,000			200,000,000
Income taxes									76,000,000			367,000,000
Segment net income (loss)	(60,000,000)	$ 15,000,000	$ 49,000,000	$ 239,000,000	110,000,000	$ 4,000,000			114,000,000			243,000,000
Gross property additions									632,000,000			1,508,000,000
Total assets	22,987,000,000				21,853,000,000				21,853,000,000			22,987,000,000	21,853,000,000
Segment and Related Information (Textual) [Abstract]
Loss on Plant Scherer Unit 3												0
SOUTHERN Co GAS | Operating Segments
Financial data for business segments
Operating revenues									1,751,000,000			4,144,000,000
Depreciation and amortization									230,000,000			473,000,000
Operating Income (Loss)									240,000,000			702,000,000
Earnings from equity method investments									58,000,000			103,000,000
Interest expense									125,000,000			198,000,000
Income taxes									71,000,000			263,000,000
Segment net income (loss)									116,000,000			383,000,000
Gross property additions									621,000,000			1,474,000,000
Total assets	24,842,000,000				24,875,000,000				24,875,000,000			24,842,000,000	24,875,000,000
SOUTHERN Co GAS | Operating Segments | Gas Distribution Operations
Financial data for business segments
Operating revenues									1,342,000,000			3,207,000,000
Depreciation and amortization									185,000,000			391,000,000
Operating Income (Loss)									222,000,000			650,000,000
Earnings from equity method investments									0			0
Interest expense									105,000,000			153,000,000
Income taxes									51,000,000			178,000,000
Segment net income (loss)									77,000,000			353,000,000
Gross property additions									561,000,000			1,330,000,000
Total assets	19,358,000,000				19,453,000,000				19,453,000,000			19,358,000,000	19,453,000,000
SOUTHERN Co GAS | Operating Segments | Gas Marketing Services
Financial data for business segments
Operating revenues									354,000,000			860,000,000
Depreciation and amortization									35,000,000			62,000,000
Operating Income (Loss)									27,000,000			113,000,000
Earnings from equity method investments									0			0
Interest expense									1,000,000			5,000,000
Income taxes									7,000,000			24,000,000
Segment net income (loss)									19,000,000			84,000,000
Gross property additions									5,000,000			9,000,000
Total assets	2,147,000,000				2,084,000,000				2,084,000,000			2,147,000,000	2,084,000,000
SOUTHERN Co GAS | Operating Segments | Wholesale Gas Services
Financial data for business segments
Operating revenues									24,000,000			6,000,000
Cost of revenue									6,116,000,000			6,627,000,000
Depreciation and amortization									1,000,000			2,000,000
Operating Income (Loss)									(2,000,000)			(51,000,000)
Earnings from equity method investments									0			0
Interest expense									3,000,000			7,000,000
Income taxes									(3,000,000)			0
Segment net income (loss)									0			(57,000,000)
Gross property additions									1,000,000			1,000,000
Total assets	1,096,000,000				1,127,000,000				1,127,000,000			1,096,000,000	1,127,000,000
SOUTHERN Co GAS | Operating Segments | Gas Midstream Operations
Financial data for business segments
Operating revenues									31,000,000			71,000,000
Depreciation and amortization									9,000,000			18,000,000
Operating Income (Loss)									(7,000,000)			(10,000,000)
Earnings from equity method investments									58,000,000			103,000,000
Interest expense									16,000,000			33,000,000
Income taxes									16,000,000			61,000,000
Segment net income (loss)									20,000,000			3,000,000
Gross property additions									54,000,000			134,000,000
Total assets	2,241,000,000				2,211,000,000				2,211,000,000			2,241,000,000	2,211,000,000
SOUTHERN Co GAS | Operating Segments | All Other [Member]
Financial data for business segments
Operating revenues									3,000,000			10,000,000
Depreciation and amortization									8,000,000			28,000,000
Operating Income (Loss)									(43,000,000)			(37,000,000)
Earnings from equity method investments									2,000,000			3,000,000
Interest expense									(44,000,000)			2,000,000
Income taxes									5,000,000			104,000,000
Segment net income (loss)									(2,000,000)			(140,000,000)
Gross property additions									11,000,000			34,000,000
Total assets	12,184,000,000				11,145,000,000				11,145,000,000			12,184,000,000	11,145,000,000
SOUTHERN Co GAS | Intersegment Eliminations
Financial data for business segments
Operating revenues									(102,000,000)			(234,000,000)
Depreciation and amortization									0			0
Operating Income (Loss)									0			0
Earnings from equity method investments									0			0
Interest expense									0			0
Income taxes									0			0
Segment net income (loss)									0			0
Gross property additions									0			0
Total assets	$ (14,039,000,000)				$ (14,167,000,000)				(14,167,000,000)			(14,039,000,000)	$ (14,167,000,000)
Predecessor | SOUTHERN Co GAS
Financial data for business segments
Operating revenues							571,000,000	1,334,000,000		$ 1,905,000,000				3,941,000,000
Depreciation and amortization										206,000,000				397,000,000
Operating Income (Loss)							(27,000,000)	348,000,000		321,000,000				746,000,000
Earnings from equity method investments										2,000,000				6,000,000
EBIT							(23,000,000)	351,000,000		328,000,000				761,000,000
Interest expense										96,000,000				175,000,000
Income taxes										87,000,000				213,000,000
Segment net income (loss)							$ (51,000,000)	$ 182,000,000		131,000,000				353,000,000
Gross property additions										548,000,000				1,027,000,000
Total assets														14,754,000,000
Predecessor | SOUTHERN Co GAS | Operating Segments
Financial data for business segments
Operating revenues										2,003,000,000				4,141,000,000
Depreciation and amortization										199,000,000				380,000,000
Operating Income (Loss)										382,000,000				809,000,000
EBIT										388,000,000				820,000,000
Gross property additions										532,000,000				993,000,000
Total assets														14,832,000,000
Predecessor | SOUTHERN Co GAS | Operating Segments | Gas Distribution Operations
Financial data for business segments
Operating revenues										1,575,000,000				3,049,000,000
Depreciation and amortization										178,000,000				336,000,000
Operating Income (Loss)										351,000,000				571,000,000
EBIT										353,000,000				581,000,000
Gross property additions										484,000,000				957,000,000
Total assets														12,519,000,000
Predecessor | SOUTHERN Co GAS | Operating Segments | Gas Marketing Services
Financial data for business segments
Operating revenues										435,000,000				835,000,000
Depreciation and amortization										11,000,000				25,000,000
Operating Income (Loss)										109,000,000				152,000,000
EBIT										109,000,000				152,000,000
Gross property additions										4,000,000				7,000,000
Total assets														686,000,000
Predecessor | SOUTHERN Co GAS | Operating Segments | Wholesale Gas Services
Financial data for business segments
Operating revenues										(32,000,000)				202,000,000
Cost of revenue										2,675,000,000				6,492,000,000
Depreciation and amortization										1,000,000				1,000,000
Operating Income (Loss)										(69,000,000)				112,000,000
EBIT										(68,000,000)				110,000,000
Gross property additions										1,000,000				2,000,000
Total assets														935,000,000
Predecessor | SOUTHERN Co GAS | Operating Segments | Gas Midstream Operations
Financial data for business segments
Operating revenues										25,000,000				55,000,000
Depreciation and amortization										9,000,000				18,000,000
Operating Income (Loss)										(9,000,000)				(26,000,000)
EBIT										(6,000,000)				(23,000,000)
Gross property additions										43,000,000				27,000,000
Total assets														692,000,000
Predecessor | SOUTHERN Co GAS | Operating Segments | All Other [Member]
Financial data for business segments
Operating revenues										4,000,000				11,000,000
Depreciation and amortization										7,000,000				17,000,000
Operating Income (Loss)										(61,000,000)				(63,000,000)
EBIT										(60,000,000)				(59,000,000)
Gross property additions										16,000,000				34,000,000
Total assets														9,662,000,000
Predecessor | SOUTHERN Co GAS | Intersegment Eliminations
Financial data for business segments
Operating revenues										(102,000,000)				(211,000,000)
Depreciation and amortization										0				0
Operating Income (Loss)										0				0
EBIT										0				0
Gross property additions										0				0
Total assets														(9,740,000,000)
Third Party Gross Revenues | SOUTHERN Co GAS | Operating Segments | Wholesale Gas Services
Financial data for business segments
Operating revenues									5,807,000,000			6,152,000,000
Third Party Gross Revenues | Predecessor | SOUTHERN Co GAS | Operating Segments | Wholesale Gas Services
Financial data for business segments
Operating revenues										2,500,000,000				6,286,000,000
Intercompany Revenues | SOUTHERN Co GAS | Operating Segments | Wholesale Gas Services
Financial data for business segments
Operating revenues									333,000,000			481,000,000
Intercompany Revenues | Predecessor | SOUTHERN Co GAS | Operating Segments | Wholesale Gas Services
Financial data for business segments
Operating revenues										143,000,000				408,000,000
Total Gross Revenues | SOUTHERN Co GAS | Operating Segments | Wholesale Gas Services
Financial data for business segments
Operating revenues									$ 6,140,000,000			$ 6,633,000,000
Total Gross Revenues | Predecessor | SOUTHERN Co GAS | Operating Segments | Wholesale Gas Services
Financial data for business segments
Operating revenues										$ 2,643,000,000				$ 6,694,000,000