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, 2018		1,035	1						31					9					1
Beginning balance at Dec. 31, 2018	$ 29,039	$ 5,164	$ (38)	$ 11,094	$ 8,706	$ (203)	$ 4,316	$ 7,477	$ 1,222	$ 3,508	$ 2,775	$ (28)	$ 14,323	$ 398	$ 10,322	$ 3,612	$ (9)	$ 1,609	$ 38	$ 4,546	$ (2,971)	$ (4)	$ 7,284	$ 2,968	$ 1,600	$ 1,352	$ 16	$ 4,316	$ 8,570	$ 8,856	$ (312)	$ 26
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	2,055				2,084		(29)
Net income (loss)													311			311		37			37		27	56		56		(29)	270		270
Net income								217			217
Return of capital to parent company																		(38)		(38)
Capital contributions from parent company								1,236		1,236			29		29			2		2			1	1	1				17	17
Other comprehensive income (loss)								1				1	1				1						(4)	(4)			(4)		(1)			(1)
Stock issued (in shares)		6
Stock issued	224	$ 28		196
Stock-based compensation	24			24
Cash dividends on common stock	(623)				(623)			(211)			(211)		(394)			(394)							(51)	(51)		(51)			(118)		(118)
Contributions from noncontrolling interests	3						3																3					3
Distributions to noncontrolling interests	(41)						(41)																(41)					(41)
Other	6		$ (2)	7			1						(1)		(1)								(1)	(2)	(1)	(1)		1
Ending balance (in shares) at Mar. 31, 2019		1,041	1						31					9					1
Ending balance at Mar. 31, 2019	30,687	$ 5,192	$ (40)	11,321	10,167	(203)	4,250	8,720	$ 1,222	4,744	2,781	(27)	14,269	$ 398	10,350	3,529	(8)	1,610	$ 38	4,510	(2,934)	(4)	7,218	2,968	1,600	1,356	12	4,250	8,738	8,873	(160)	25
Beginning balance (in shares) at Dec. 31, 2018		1,035	1						31					9					1
Beginning balance at Dec. 31, 2018	29,039	$ 5,164	$ (38)	11,094	8,706	(203)	4,316	7,477	$ 1,222	3,508	2,775	(28)	14,323	$ 398	10,322	3,612	(9)	1,609	$ 38	4,546	(2,971)	(4)	7,284	2,968	1,600	1,352	16	4,316	8,570	8,856	(312)	26
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	4,335							993					1,598					139					342						347
Net income	4,298							982															316						347
Purchase of membership interests from noncontrolling interests	0																						0
Ending balance (in shares) at Sep. 30, 2019		1,050	1						31					9					1
Ending balance at Sep. 30, 2019	31,791	$ 5,234	$ (41)	11,512	11,087	(279)	4,278	9,086	$ 1,222	4,765	3,124	(25)	14,764	$ 398	10,408	4,028	(70)	1,639	$ 38	4,437	(2,832)	(4)	6,693	2,415	905	1,514	(4)	4,278	9,393	9,692	(318)	19
Beginning balance (in shares) at Mar. 31, 2019		1,041	1						31					9					1
Beginning balance at Mar. 31, 2019	30,687	$ 5,192	$ (40)	11,321	10,167	(203)	4,250	8,720	$ 1,222	4,744	2,781	(27)	14,269	$ 398	10,350	3,529	(8)	1,610	$ 38	4,510	(2,934)	(4)	7,218	2,968	1,600	1,356	12	4,250	8,738	8,873	(160)	25
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	928				899		29
Net income (loss)													448			448		37			37		203	174		174		29	106		106
Net income								296			296
Return of capital to parent company																		(38)		(38)			(505)	(505)	(505)
Capital contributions from parent company								23		23			20		20			8		8			7	7	7				35	35
Other comprehensive income (loss)	(35)					(35)		1				1	(27)				(27)						(8)	(8)			(8)		(3)			(3)
Stock issued (in shares)		5
Stock issued	228	$ 25		203
Stock-based compensation	11			11
Cash dividends on common stock	(646)				(646)			(211)			(211)		(394)			(394)							(52)	(52)		(52)			(117)		(117)
Contributions from noncontrolling interests	2						2																2					2
Distributions to noncontrolling interests	(47)						(47)																(47)					(47)
Other	3		$ (1)	5			(1)						0		1	(1)							0	1		1		(1)
Ending balance (in shares) at Jun. 30, 2019		1,046	1						31					9					1
Ending balance at Jun. 30, 2019	31,131	$ 5,217	$ (41)	11,540	10,420	(238)	4,233	8,829	$ 1,222	4,767	2,866	(26)	14,316	$ 398	10,371	3,582	(35)	1,617	$ 38	4,480	(2,897)	(4)	6,818	2,585	1,102	1,479	4	4,233	8,759	8,908	(171)	22
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	1,341				1,316		25
Net income (loss)	1,345							473					839			839		65			65		111	86		86		25	(29)		(29)
Net income	1,316							469			469												86						(29)
Return of capital to parent company								(2)		(2)								(43)		(43)			(250)	(250)	(250)
Capital contributions from parent company													38		38								53	53	53				784	784
Other comprehensive income (loss)	(41)					(41)		1				1	(35)				(35)						(8)	(8)			(8)		(3)			(3)
Issuance of equity units	(198)			(198)
Stock issued (in shares)		4
Stock issued	171	$ 17		154
Stock-based compensation	12			12
Cash dividends on common stock	(649)				(649)			(211)			(211)		(394)			(394)							(51)	(51)		(51)			(118)		(118)
Contributions from noncontrolling interests	63						63																63					63
Distributions to noncontrolling interests	(43)						(43)																(43)					(43)
Other	4		$ 0	4			0						0		(1)	1
Ending balance (in shares) at Sep. 30, 2019		1,050	1						31					9					1
Ending balance at Sep. 30, 2019	31,791	$ 5,234	$ (41)	11,512	11,087	(279)	4,278	9,086	$ 1,222	4,765	3,124	(25)	14,764	$ 398	10,408	4,028	(70)	1,639	$ 38	4,437	(2,832)	(4)	6,693	2,415	905	1,514	(4)	4,278	9,393	9,692	(318)	19
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)	837				868		(31)
Net income (loss)													331			331		32			32		44	75		75		(31)	275		275
Net income								280			280
Return of capital to parent company																		(37)		(37)									(2)	(2)
Capital contributions from parent company								612		612			502		502			76		76
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)
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	2,732							1,022															212						360
Purchase of membership interests from noncontrolling interests	60																						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]
Net income (loss)	617				612		5
Net income (loss)													308			308		39			39		68	63		63		5	71		71
Net income								298			298
Return of capital to parent company																		(37)		(37)
Capital contributions from parent company								1		1			1		1														200	200
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)
Contributions from noncontrolling interests	165						165																165					165
Distributions to noncontrolling interests	(70)						(70)																(70)					(70)
Other	(12)			(13)	1																		(2)	(2)	(2)	0	0	0
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]
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	1,251							444			444												74						14
Return of capital to parent company																							(4)	(4)	(4)
Capital contributions from parent company								40		40			880		880			6		6									30	30
Other comprehensive income (loss)	20					20		1				1	2				2						8	8			8		5			5
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)
Contributions from noncontrolling interests	2						2																2					2
Distributions to noncontrolling interests	(51)						(51)																(51)					(51)
Purchase of membership interests from noncontrolling interests	(55)			5			(60)																(55)	5	5			(60)
Other	1	$ 0	$ 0	0	1	(1)	1																1	0	0	0		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)