Retirement Benefits - Changes in Projected Benefit Obligations and Fair Value of Plan Assets (Details) - USD ($) $ in Millions			6 Months Ended		12 Months Ended
	Dec. 19, 2016	Sep. 12, 2016	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension plans
Change in benefit obligation
Benefit obligation at beginning of year				$ 10,542	$ 10,542	$ 10,909
Acquisitions					1,244	0
Service cost					262	257	$ 213
Interest cost					422	445	435
Benefits paid					(466)	(487)
Actuarial loss (gain)					381	(582)
Balance at end of year			$ 12,385		12,385	10,542	10,909
Change in plan assets
Fair value of plan assets at beginning of year				9,234	9,234	9,690
Acquisitions					837	0
Actual return (loss) on plan assets					902	(14)
Employer contributions	$ 900				1,076	45
Benefits paid					(466)	(487)
Fair value of plan assets at end of year			11,583		11,583	9,234	9,690
Accrued liability			(802)		(802)	(1,308)
Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year				1,989	1,989	1,986
Acquisitions					338	0
Service cost					22	23	21
Interest cost					76	78	79
Benefits paid					(119)	(102)
Actuarial loss (gain)					(16)	(38)
Plan amendments					0	34
Retiree drug subsidy					7	8
Balance at end of year			2,297		2,297	1,989	1,986
Change in plan assets
Fair value of plan assets at beginning of year				833	833	900
Acquisitions					100	0
Actual return (loss) on plan assets					58	(12)
Employer contributions					65	39
Benefits paid					(119)	(102)
Benefits paid, net of drug subsidy					(112)	(94)
Fair value of plan assets at end of year			944		944	833	900
Accrued liability			(1,353)		(1,353)	(1,156)
Georgia Power [Member] | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year				3,615	3,615	3,781
Service cost					70	73	62
Interest cost					136	154	153
Benefits paid					(164)	(188)
Actuarial loss (gain)					143	(205)
Balance at end of year			3,800		3,800	3,615	3,781
Change in plan assets
Fair value of plan assets at beginning of year				3,196	3,196	3,383
Actual return (loss) on plan assets					288	(13)
Employer contributions	287				301	14
Benefits paid					(164)	(188)
Fair value of plan assets at end of year			3,621		3,621	3,196	3,383
Accrued liability			(179)		(179)	(419)
Georgia Power [Member] | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year				854	854	864
Service cost					6	7	6
Interest cost					30	34	34
Benefits paid					(45)	(45)
Actuarial loss (gain)					(1)	(22)
Plan amendments					0	12
Retiree drug subsidy					3	4
Balance at end of year			847		847	854	864
Change in plan assets
Fair value of plan assets at beginning of year				358	358	395
Actual return (loss) on plan assets					21	(6)
Employer contributions					17	10
Benefits paid					(45)	(45)
Benefits paid, net of drug subsidy					(42)	(41)
Fair value of plan assets at end of year			354		354	358	395
Accrued liability			(493)		(493)	(496)
Alabama Power [Member] | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year				2,506	2,506	2,592
Service cost					57	59	48
Interest cost					95	106	103
Benefits paid					(109)	(120)
Actuarial loss (gain)					114	(131)
Balance at end of year			2,663		2,663	2,506	2,592
Change in plan assets
Fair value of plan assets at beginning of year				2,279	2,279	2,396
Actual return (loss) on plan assets					206	(9)
Employer contributions	129				141	12
Benefits paid					(109)	(120)
Fair value of plan assets at end of year			2,517		2,517	2,279	2,396
Accrued liability			(146)		(146)	(227)
Alabama Power [Member] | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year				505	505	503
Service cost					5	6	5
Interest cost					18	20	20
Benefits paid					(28)	(27)
Actuarial loss (gain)					(1)	(7)
Plan amendments					0	7
Retiree drug subsidy					2	3
Balance at end of year			501		501	505	503
Change in plan assets
Fair value of plan assets at beginning of year				363	363	392
Actual return (loss) on plan assets					23	(6)
Employer contributions					7	1
Benefits paid					(28)	(27)
Benefits paid, net of drug subsidy					(26)	(24)
Fair value of plan assets at end of year			367		367	363	392
Accrued liability			(134)		(134)	(142)
Gulf Power [Member]
Change in benefit obligation
Service cost					12	12	10
Interest cost					19	20	19
Gulf Power [Member] | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year				480	480	491
Service cost					12	12
Interest cost					19	20
Benefits paid					(17)	(20)
Actuarial loss (gain)					23	(23)
Balance at end of year			517		517	480	491
Change in plan assets
Fair value of plan assets at beginning of year				420	420	435
Actual return (loss) on plan assets					39	4
Employer contributions	48				49	1
Benefits paid					(17)	(20)
Fair value of plan assets at end of year			491		491	420	435
Accrued liability			(26)		(26)	(60)
Gulf Power [Member] | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year				81	81	78
Service cost					1	1	1
Interest cost					3	3	3
Benefits paid					(4)	(4)
Actuarial loss (gain)					2	(1)
Plan amendments					0	4
Balance at end of year			83		83	81	78
Change in plan assets
Fair value of plan assets at beginning of year				17	17	18
Actual return (loss) on plan assets					2	0
Employer contributions					3	3
Benefits paid					(4)	(4)
Benefits paid, net of drug subsidy					(4)	(4)
Fair value of plan assets at end of year			18		18	17	18
Accrued liability			(65)		(65)	(64)
Mississippi Power [Member]
Change in benefit obligation
Service cost					13	13	10
Interest cost					19	21	20
Mississippi Power [Member] | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year				500	500	513
Service cost					13	13
Interest cost					19	21
Benefits paid					(20)	(22)
Actuarial loss (gain)					22	(25)
Balance at end of year			534		534	500	513
Change in plan assets
Fair value of plan assets at beginning of year				430	430	446
Actual return (loss) on plan assets					39	4
Employer contributions	$ 47				50	2
Benefits paid					(20)	(22)
Fair value of plan assets at end of year			499		499	430	446
Accrued liability			(35)		(35)	(70)
Mississippi Power [Member] | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year				97	97	96
Service cost					1	1	1
Interest cost					3	4	4
Benefits paid					(6)	(5)
Actuarial loss (gain)					1	(1)
Plan amendments					0	1
Retiree drug subsidy					1	1
Balance at end of year			97		97	97	96
Change in plan assets
Fair value of plan assets at beginning of year				23	23	24
Actual return (loss) on plan assets					1	0
Employer contributions					4	3
Benefits paid					(6)	(5)
Benefits paid, net of drug subsidy					(5)	(4)
Fair value of plan assets at end of year			23		23	23	24
Accrued liability			(74)		(74)	(74)
Successor [Member] | Southern Company Gas [Member]
Change in plan assets
Employee contributions			1
Successor [Member] | Southern Company Gas [Member] | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year			1,244
Service cost			15
Interest cost			20
Benefits paid			(31)
Actuarial loss (gain)			(115)
Balance at end of year			1,133	1,244	1,133
Change in plan assets
Fair value of plan assets at beginning of year			837
Actual return (loss) on plan assets			48
Employer contributions		$ 125	129
Benefits paid			(31)
Fair value of plan assets at end of year			983	837	983
Accrued liability			(150)		(150)
Successor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year			338
Service cost			1
Interest cost			5
Benefits paid			(11)
Actuarial loss (gain)			(26)
Retiree drug subsidy			0
Balance at end of year			308	338	308
Change in plan assets
Fair value of plan assets at beginning of year			100
Actual return (loss) on plan assets			4
Employee contributions			1
Employer contributions			11
Benefits paid			(11)
Benefits paid, net of drug subsidy			(11)
Fair value of plan assets at end of year			105	100	105
Accrued liability			(203)		(203)
Predecessor [Member] | Southern Company Gas [Member]
Change in plan assets
Employee contributions				1		1
Predecessor [Member] | Southern Company Gas [Member] | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year			1,244	1,067	1,067	1,098
Service cost				13		28	24
Interest cost				21		45	47
Benefits paid				(26)		(49)
Actuarial loss (gain)				169		(55)
Balance at end of year				1,244		1,067	1,098
Change in plan assets
Fair value of plan assets at beginning of year			837	847	847	906
Actual return (loss) on plan assets				15		(12)
Employer contributions				1		2
Benefits paid				(26)		(49)
Fair value of plan assets at end of year				837		847	906
Accrued liability				(407)		(220)
Predecessor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year			338	318	318	334
Service cost				1		2	2
Interest cost				5		13	15
Benefits paid				(11)		(20)
Actuarial loss (gain)				24		(13)
Retiree drug subsidy				0		1
Balance at end of year				338		318	334
Change in plan assets
Fair value of plan assets at beginning of year			$ 100	99	$ 99	99
Actual return (loss) on plan assets				1		1
Employee contributions				1		1
Employer contributions				10		17
Benefits paid				(11)		(20)
Benefits paid, net of drug subsidy				(11)		(20)
Fair value of plan assets at end of year				100		99	$ 99
Accrued liability				$ (238)		$ (219)