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 Paid-In Capital	SOUTHERN POWER CO Retained Earnings (Accumulated Deficit)	SOUTHERN POWER CO Accumulated Other Comprehensive Income (Loss)	SOUTHERN POWER CO Total Common Stockholders' Equity	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, 2017		1,009	1						31					9					1
Beginning balance at Dec. 31, 2017	$ 25,528	$ 5,038	$ (36)	$ 10,469	$ 8,885	$ (189)	$ 1,361	$ 6,829	$ 1,222	$ 2,986	$ 2,647	$ (26)	$ 11,931	$ 398	$ 7,328	$ 4,215	$ (10)	$ 1,358	$ 38	$ 4,529	$ (3,205)	$ (4)	$ 6,498	$ 3,662	$ 1,478	$ (2)	$ 5,138	$ 1,360	$ 9,022	$ 9,214	$ (212)	$ 20
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	938				938			225			225		352			352		(7)			(7)		121		121		121		279		279
Other comprehensive income (loss)	30					30		1				1	1				1	(1)				(1)	24			24	24		2			2
Stock issued (in shares)		4
Stock issued	113	$ 16		97
Stock-based compensation	36			36
Cash dividends on common stock	(586)				(586)			(202)			(202)		(339)			(339)							(78)		(78)		(78)		(118)		(118)
Contributions from noncontrolling interests	9						9																9					9
Distributions to noncontrolling interests	(13)						(13)																(13)					(13)
Net income (loss) attributable to noncontrolling interests	(6)						(6)																(6)					(6)
Other	(24)		$ (2)	1	20	(41)	(2)	(6)				(6)	(1)		1		(2)	(1)			(1)		2		(2)	5	3	(1)	0		(4)	4
Capital contributions from parent company								488		488			1,476		1,476			2		2			1	1			1		14	14
Ending balance (in shares) at Mar. 31, 2018		1,013	1						31					9					1
Ending balance at Mar. 31, 2018	26,025	$ 5,054	$ (38)	10,603	9,257	(200)	1,349	7,335	$ 1,222	3,474	2,670	(31)	13,420	$ 398	8,805	4,228	(11)	1,351	$ 38	4,531	(3,213)	(5)	6,558	3,663	1,519	27	5,209	1,349	9,199	9,228	(55)	26
Beginning balance (in shares) at Dec. 31, 2017		1,009	1						31					9					1
Beginning balance at Dec. 31, 2017	25,528	$ 5,038	$ (36)	10,469	8,885	(189)	1,361	6,829	$ 1,222	2,986	2,647	(26)	11,931	$ 398	7,328	4,215	(10)	1,358	$ 38	4,529	(3,205)	(4)	6,498	3,662	1,478	(2)	5,138	1,360	9,022	9,214	(212)	20
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	784							484										39					143						248
Ending balance (in shares) at Jun. 30, 2018		1,015	1						31					9					1
Ending balance at Jun. 30, 2018	26,692	$ 5,066	$ (39)	10,303	8,494	(188)	3,056	7,401	$ 1,222	3,480	2,729	(30)	12,702	$ 398	8,834	3,480	(10)	1,398	$ 38	4,531	(3,166)	(5)	7,574	3,023	1,464	31	4,518	3,056	9,061	9,236	(202)	27
Beginning balance (in shares) at Mar. 31, 2018		1,013	1						31					9					1
Beginning balance at Mar. 31, 2018	26,025	$ 5,054	$ (38)	10,603	9,257	(200)	1,349	7,335	$ 1,222	3,474	2,670	(31)	13,420	$ 398	8,805	4,228	(11)	1,351	$ 38	4,531	(3,213)	(5)	6,558	3,663	1,519	27	5,209	1,349	9,199	9,228	(55)	26
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(154)				(154)			259			259		(396)			(396)		46			46		22		22		22		(31)		(31)
Other comprehensive income (loss)	12					12		1				1	1				1						4			4	4		1			1
Stock issued (in shares)		2
Stock issued	109	$ 12		97
Stock-based compensation	12			12
Cash dividends on common stock	(607)				(607)			(200)			(200)		(352)			(352)							(78)		(78)		(78)		(117)		(117)
Contributions from noncontrolling interests	22						22																22					22
Distributions to noncontrolling interests	(29)						(29)																(29)					(29)
Net income (loss) attributable to noncontrolling interests	23						23																23					23
Other	(4)		$ (1)	(2)	(2)		1	1		1								1			1		2		1		1	1	1		1
Sale of noncontrolling interests	1,283			(407)			1,690																1,283	(407)			(407)	1,690
Capital contributions from parent company								5		5			29		29								17	17			17		8	8
Return of capital to parent company																							(250)	(250)			(250)
Ending balance (in shares) at Jun. 30, 2018		1,015	1						31					9					1
Ending balance at Jun. 30, 2018	26,692	$ 5,066	$ (39)	10,303	8,494	(188)	3,056	7,401	$ 1,222	3,480	2,729	(30)	12,702	$ 398	8,834	3,480	(10)	1,398	$ 38	4,531	(3,166)	(5)	7,574	3,023	1,464	31	4,518	3,056	9,061	9,236	(202)	27
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	1,600	1,352	16	2,968	4,316	8,570	8,856	(312)	26
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	2,084				2,084			217			217		311			311		37			37		56		56		56		270		270
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)
Net income (loss) attributable to noncontrolling interests	(29)						(29)																(29)					(29)
Other (in shares)			0
Other	6		$ (2)	7	0		1						(1)		(1)		0						(1)	(1)	(1)		(2)	1
Capital contributions from parent company								1,236		1,236			29		29			2		2			1	1			1		17	17
Return of capital to parent company																		(38)		(38)
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	1,600	1,356	12	2,968	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	1,600	1,352	16	2,968	4,316	8,570	8,856	(312)	26
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	2,982							513										74					230						376
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	1,102	1,479	4	2,585	4,233	8,759	8,908	(171)	22
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	1,600	1,356	12	2,968	4,250	8,738	8,873	(160)	25
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	899				899			296			296		448			448		37			37		174		174		174		106		106
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)
Net income (loss) attributable to noncontrolling interests	$ 29						29																29					29
Other (in shares)	0
Other	$ 3		$ (1)	5	0		(1)						0		1	(1)							0		1		1	(1)
Capital contributions from parent company								23		23			20		20			8		8			7	7			7		35	35
Return of capital to parent company																							(505)	(505)			(505)
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	$ 1,102	$ 1,479	$ 4	$ 2,585	$ 4,233	$ 8,759	$ 8,908	$ (171)	$ 22