Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Treasury	Paid-In Capital	Retained Earnings (Accumulated Deficit)	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interests	ALABAMA POWER CO	ALABAMA POWER CO Common Stock	ALABAMA POWER CO Paid-In Capital	ALABAMA POWER CO Retained Earnings (Accumulated Deficit)	ALABAMA POWER CO Accumulated Other Comprehensive Income (Loss)	GEORGIA POWER CO	GEORGIA POWER CO Common Stock	GEORGIA POWER CO Paid-In Capital	GEORGIA POWER CO Retained Earnings (Accumulated Deficit)	GEORGIA POWER CO Accumulated Other Comprehensive Income (Loss)	MISSISSIPPI POWER CO	MISSISSIPPI POWER CO Common Stock	MISSISSIPPI POWER CO Paid-In Capital	MISSISSIPPI POWER CO Retained Earnings (Accumulated Deficit)	MISSISSIPPI POWER CO Accumulated Other Comprehensive Income (Loss)	SOUTHERN POWER CO	SOUTHERN POWER CO Total Common Stockholders' Equity	SOUTHERN POWER CO Paid-In Capital	SOUTHERN POWER CO Retained Earnings (Accumulated Deficit)	SOUTHERN POWER CO Accumulated Other Comprehensive Income (Loss)	SOUTHERN POWER CO Noncontrolling Interests	SOUTHERN Co GAS	SOUTHERN Co GAS Paid-In Capital	SOUTHERN Co GAS Retained Earnings (Accumulated Deficit)	SOUTHERN Co GAS Accumulated Other Comprehensive Income (Loss)
Beginning balance (in shares) at Dec. 31, 2019		1,054	1						31					9					1
Beginning balance at Dec. 31, 2019	$ 31,759	$ 5,257	$ (42)	$ 11,734	$ 10,877	$ (321)	$ 4,254	$ 8,955	$ 1,222	$ 4,755	$ 3,001	$ (23)	$ 15,065	$ 398	$ 10,962	$ 3,756	$ (51)	$ 1,652	$ 38	$ 4,449	$ (2,832)	$ (3)	$ 6,622	$ 2,368	$ 909	$ 1,485	$ (26)	$ 4,254	$ 9,506	$ 9,697	$ (198)	$ 7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Consolidated net income (loss)	837				868		(31)
Net income (loss)													331			331		32			32		44	75		75		(31)	275		275
Net income after dividends on preferred stock								280			280
Capital contributions from parent company								612		612			502		502			76		76
Return of capital to parent company																		(37)		(37)									(2)	(2)
Other comprehensive income (loss)	(47)					(47)		1				1	(1)				(1)						(33)	(33)			(33)		(15)			(15)
Stock issued (in shares)		3
Stock issued	52	$ 9		43
Stock-based compensation	5			5
Cash dividends on common stock	(655)				(655)			(239)			(239)		(385)			(385)							(50)	(50)		(50)			(133)		(133)
Capital contributions from noncontrolling interests	16						16																16					16
Distributions to noncontrolling interests	(48)						(48)																(48)					(48)
Other	(3)		$ (2)		(2)	1												(1)		(1)
Ending balance (in shares) at Mar. 31, 2020		1,057	1						31					9					1
Ending balance at Mar. 31, 2020	31,916	$ 5,266	$ (44)	11,782	11,088	(367)	4,191	9,609	$ 1,222	5,367	3,042	(22)	15,512	$ 398	11,464	3,702	(52)	1,722	$ 38	4,487	(2,800)	(3)	6,551	2,360	909	1,510	(59)	4,191	9,631	9,695	(56)	(8)
Beginning balance (in shares) at Dec. 31, 2019		1,054	1						31					9					1
Beginning balance at Dec. 31, 2019	31,759	$ 5,257	$ (42)	11,734	10,877	(321)	4,254	8,955	$ 1,222	4,755	3,001	(23)	15,065	$ 398	10,962	3,756	(51)	1,652	$ 38	4,449	(2,832)	(3)	6,622	2,368	909	1,485	(26)	4,254	9,506	9,697	(198)	7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	2,746							1,033					1,411					138					215						360
Net income after dividends on preferred stock	2,732																												360
Purchase of membership interests from noncontrolling interests																							60
Ending balance (in shares) at Sep. 30, 2020		1,057	1						31					9					1
Ending balance at Sep. 30, 2020	32,500	$ 5,267	$ (44)	11,810	11,600	(344)	4,211	9,915	$ 1,222	5,408	3,305	(20)	16,706	$ 398	12,345	4,011	(48)	1,760	$ 38	4,456	(2,731)	(3)	6,611	2,400	908	1,546	(54)	4,211	9,685	9,925	(237)	(3)
Beginning balance (in shares) at Mar. 31, 2020		1,057	1						31					9					1
Beginning balance at Mar. 31, 2020	31,916	$ 5,266	$ (44)	11,782	11,088	(367)	4,191	9,609	$ 1,222	5,367	3,042	(22)	15,512	$ 398	11,464	3,702	(52)	1,722	$ 38	4,487	(2,800)	(3)	6,551	2,360	909	1,510	(59)	4,191	9,631	9,695	(56)	(8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Consolidated net income (loss)	617				612		5
Net income (loss)													308			308		39			39		68	63		63		5	71		71
Net income after dividends on preferred stock								298			298
Capital contributions from parent company								1		1			1		1														200	200
Return of capital to parent company																		(37)		(37)
Other comprehensive income (loss)	4					4		1				1	2				2						(3)	(3)			(3)
Stock issued	7			7
Stock-based compensation	11			11
Cash dividends on common stock	(677)				(677)			(239)			(239)		(386)			(386)							(50)	(50)		(50)			(133)		(133)
Capital contributions from noncontrolling interests	165						165																165					165
Distributions to noncontrolling interests	(70)						(70)																(70)					(70)
Other	(12)			(13)	1																		(2)	(2)	(2)
Ending balance (in shares) at Jun. 30, 2020		1,057	1						31					9					1
Ending balance at Jun. 30, 2020	31,961	$ 5,266	$ (44)	11,787	11,024	(363)	4,291	9,670	$ 1,222	5,368	3,101	(21)	15,437	$ 398	11,465	3,624	(50)	1,724	$ 38	4,450	(2,761)	(3)	6,659	2,368	907	1,523	(62)	4,291	9,769	9,895	(118)	(8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Consolidated net income (loss)	1,279				1,251		28
Net income (loss)	1,283							448					773			773		67			67		102	74		74		28	14		14
Net income after dividends on preferred stock	1,251							444			444																		14
Capital contributions from parent company								40		40			880		880			6		6									30	30
Return of capital to parent company																							(4)	(4)	(4)
Other comprehensive income (loss)	20					20		1				1	2				2						8	8			8		5			5
Stock issued (in shares)		0
Stock issued	4	$ 1		3
Stock-based compensation	15			15
Cash dividends on common stock	(676)				(676)			(240)			(240)		(386)			(386)		(37)			(37)		(51)	(51)		(51)			(133)		(133)
Capital contributions from noncontrolling interests	2						2																2					2
Distributions to noncontrolling interests	(51)						(51)																(51)					(51)
Purchase of membership interests from noncontrolling interests																								5	5
Purchase of membership interests from noncontrolling interests	(55)						(60)																(55)					(60)
Other	1		$ 0	0	1	(1)	1
Ending balance (in shares) at Sep. 30, 2020		1,057	1						31					9					1
Ending balance at Sep. 30, 2020	32,500	$ 5,267	$ (44)	11,810	11,600	(344)	4,211	9,915	$ 1,222	5,408	3,305	(20)	16,706	$ 398	12,345	4,011	(48)	1,760	$ 38	4,456	(2,731)	(3)	6,611	2,400	908	1,546	(54)	4,211	9,685	9,925	(237)	(3)
Beginning balance (in shares) at Dec. 31, 2020		1,058	1						31					9					1
Beginning balance at Dec. 31, 2020	32,234	$ 5,268	$ (46)	11,834	11,311	(395)	4,262	9,810	$ 1,222	5,413	3,194	(19)	16,501	$ 398	12,361	3,789	(47)	1,742	$ 38	4,460	(2,754)	(2)	6,631	2,369	914	1,522	(67)	4,262	9,767	9,930	(141)	(22)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Consolidated net income (loss)	1,103				1,135		(32)
Net income (loss)													351			351		45			45		65	97		97		(32)	398		398
Net income after dividends on preferred stock								359			359
Capital contributions from parent company								602		602			332		332			100		100									57	57
Return of capital to parent company																							(271)	(271)	(271)
Other comprehensive income (loss)	28					28		1				1	2				2						16	16			16		4			4
Stock issued (in shares)		2
Stock issued	14	$ 5		9
Stock-based compensation	9			9
Cash dividends on common stock	(678)				(678)			(246)			(246)		(412)			(412)		(39)			(39)		(51)	(51)		(51)			(132)		(132)
Capital contributions from noncontrolling interests	403						403																403					403
Distributions to noncontrolling interests	(46)						(46)																(46)					(46)
Other	1			2			(1)																(3)	(2)	(2)	1	(1)	(1)
Ending balance (in shares) at Mar. 31, 2021		1,060	1						31					9					1
Ending balance at Mar. 31, 2021	33,068	$ 5,273	$ (46)	11,854	11,768	(367)	4,586	10,526	$ 1,222	6,015	3,307	(18)	16,774	$ 398	12,693	3,728	(45)	1,848	$ 38	4,560	(2,748)	(2)	6,744	2,158	641	1,569	(52)	4,586	10,094	9,987	125	(18)
Beginning balance (in shares) at Dec. 31, 2020		1,058	1						31					9					1
Beginning balance at Dec. 31, 2020	32,234	$ 5,268	$ (46)	11,834	11,311	(395)	4,262	9,810	$ 1,222	5,413	3,194	(19)	16,501	$ 398	12,361	3,789	(47)	1,742	$ 38	4,460	(2,754)	(2)	6,631	2,369	914	1,522	(67)	4,262	9,767	9,930	(141)	(22)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	2,592							1,200					1,030					133					184						389
Net income after dividends on preferred stock	2,608																												389
Purchase of membership interests from noncontrolling interests																							0
Ending balance (in shares) at Sep. 30, 2021		1,061	1						31					9					1
Ending balance at Sep. 30, 2021	33,219	$ 5,278	$ (46)	11,932	11,844	(319)	4,530	10,903	$ 1,222	6,050	3,644	(13)	17,361	$ 398	13,423	3,582	(42)	1,863	$ 38	4,565	(2,739)	(1)	6,707	2,177	643	1,581	(47)	4,530	9,876	10,014	(149)	11
Beginning balance (in shares) at Mar. 31, 2021		1,060	1						31					9					1
Beginning balance at Mar. 31, 2021	33,068	$ 5,273	$ (46)	11,854	11,768	(367)	4,586	10,526	$ 1,222	6,015	3,307	(18)	16,774	$ 398	12,693	3,728	(45)	1,848	$ 38	4,560	(2,748)	(2)	6,744	2,158	641	1,569	(52)	4,586	10,094	9,987	125	(18)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Consolidated net income (loss)	372				372
Net income (loss)													143			143		38			38		36	36		36			(65)		(65)
Net income after dividends on preferred stock								331			331
Capital contributions from parent company								26		26			40		40			2		2									25	25
Other comprehensive income (loss)	12					12		1				1	1				1						(3)	(3)			(3)		8			8
Stock issued	10	$ 1		9
Stock-based compensation	22			22
Cash dividends on common stock	(699)				(699)			(246)			(246)		(412)			(412)		(39)			(39)		(51)	(51)		(51)			(133)		(133)
Capital contributions from noncontrolling interests	29						29																29					29
Distributions to noncontrolling interests	(68)						(68)																(68)					(68)
Other	0		$ (2)	1	1			(1)			(1)							0			(1)	1	3	3	2		1
Ending balance (in shares) at Jun. 30, 2021		1,060	1						31					9					1
Ending balance at Jun. 30, 2021	32,746	$ 5,274	$ (48)	11,886	11,442	(355)	4,547	10,637	$ 1,222	6,041	3,391	(17)	16,546	$ 398	12,733	3,459	(44)	1,849	$ 38	4,562	(2,750)	(1)	6,690	2,143	643	1,554	(54)	4,547	9,929	10,012	(73)	(10)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Consolidated net income (loss)	1,106				1,101		5
Net income (loss)	1,110							503					536			536		50			50		83	78		78		5	56		56
Net income after dividends on preferred stock	1,101							499			499																		56
Capital contributions from parent company								9		9			690		690			3		3									2	2
Other comprehensive income (loss)	36					36		5				5	2				2						7	7			7		21			21
Stock issued (in shares)		1
Stock issued	38	$ 4		34
Stock-based compensation	22			22
Cash dividends on common stock	(700)				(700)			(246)			(246)		(413)			(413)		(39)			(39)		(51)	(51)		(51)			(132)		(132)
Capital contributions from noncontrolling interests	72						72																73					73
Distributions to noncontrolling interests	(94)						(94)																(95)					(95)
Other	(7)		$ 2	(10)	1	0	0	(1)	$ 0	0	0	(1)											1					1
Ending balance (in shares) at Sep. 30, 2021		1,061	1						31					9					1
Ending balance at Sep. 30, 2021	$ 33,219	$ 5,278	$ (46)	$ 11,932	$ 11,844	$ (319)	$ 4,530	$ 10,903	$ 1,222	$ 6,050	$ 3,644	$ (13)	$ 17,361	$ 398	$ 13,423	$ 3,582	$ (42)	$ 1,863	$ 38	$ 4,565	$ (2,739)	$ (1)	$ 6,707	$ 2,177	$ 643	$ 1,581	$ (47)	$ 4,530	$ 9,876	$ 10,014	$ (149)	$ 11