Retirement Benefits - Changes in Projected Benefit Obligations and Fair Value of Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Plans [Member]
Change in benefit obligation
Benefit obligation at beginning of year	$ 10,909	$ 8,863
Service cost	257	213	$ 232
Interest cost	445	435	389
Benefits paid	(487)	(382)
Actuarial loss (gain)	(582)	1,780
Balance at end of year	10,542	10,909	8,863
Change in plan assets
Fair value of plan assets at beginning of year	9,690	8,733
Actual return (loss) on plan assets	(14)	797
Employer contributions	45	542
Fair value of plan assets at end of year	9,234	9,690	8,733
Accrued liability	(1,308)	(1,219)
Other Postretirement Benefits [Member]
Change in benefit obligation
Benefit obligation at beginning of year	1,986	1,682
Service cost	23	21	24
Interest cost	78	79	74
Benefits paid	(102)	(102)
Plan amendments	34	(2)
Actuarial loss (gain)	(38)	300
Retiree drug subsidy	8	8
Balance at end of year	1,989	1,986	1,682
Change in plan assets
Fair value of plan assets at beginning of year	900	901
Actual return (loss) on plan assets	(12)	54
Employer contributions	39	39
Benefits paid, net of drug subsidy	(94)	(94)
Fair value of plan assets at end of year	833	900	901
Accrued liability	(1,156)	(1,086)
Georgia Power [Member] | Pension Plans [Member]
Change in benefit obligation
Benefit obligation at beginning of year	3,781	3,116
Service cost	73	62	69
Interest cost	154	153	138
Benefits paid	(188)	(149)
Actuarial loss (gain)	(205)	599
Balance at end of year	3,615	3,781	3,116
Change in plan assets
Fair value of plan assets at beginning of year	3,383	3,085
Actual return (loss) on plan assets	(13)	285
Employer contributions	14	162
Fair value of plan assets at end of year	3,196	3,383	3,085
Accrued liability	(419)	(398)
Georgia Power [Member] | Other Postretirement Benefits [Member]
Change in benefit obligation
Benefit obligation at beginning of year	864	723
Service cost	7	6	7
Interest cost	34	34	31
Benefits paid	(45)	(44)
Plan amendments	12	0
Actuarial loss (gain)	(22)	142
Retiree drug subsidy	4	3
Balance at end of year	854	864	723
Change in plan assets
Fair value of plan assets at beginning of year	395	407
Actual return (loss) on plan assets	(6)	21
Employer contributions	10	8
Benefits paid, net of drug subsidy	(41)	(41)
Fair value of plan assets at end of year	358	395	407
Accrued liability	(496)	(469)
Alabama Power [Member] | Pension Plans [Member]
Change in benefit obligation
Benefit obligation at beginning of year	2,592	2,112
Service cost	59	48	52
Interest cost	106	103	93
Benefits paid	(120)	(100)
Actuarial loss (gain)	(131)	429
Balance at end of year	2,506	2,592	2,112
Change in plan assets
Fair value of plan assets at beginning of year	2,396	2,278
Actual return (loss) on plan assets	(9)	207
Employer contributions	12	11
Fair value of plan assets at end of year	2,279	2,396	2,278
Accrued liability	(227)	(196)
Alabama Power [Member] | Other Postretirement Benefits [Member]
Change in benefit obligation
Benefit obligation at beginning of year	503	431
Service cost	6	5	6
Interest cost	20	20	19
Benefits paid	(27)	(27)
Plan amendments	7	0
Actuarial loss (gain)	(7)	71
Retiree drug subsidy	3	3
Balance at end of year	505	503	431
Change in plan assets
Fair value of plan assets at beginning of year	392	389
Actual return (loss) on plan assets	(6)	23
Employer contributions	1	4
Benefits paid, net of drug subsidy	(24)	(24)
Fair value of plan assets at end of year	363	392	389
Accrued liability	(142)	(111)
Gulf Power [Member]
Change in benefit obligation
Service cost	12	10	11
Interest cost	20	19	17
Gulf Power [Member] | Pension Plans [Member]
Change in benefit obligation
Benefit obligation at beginning of year	491	395
Service cost	12	10
Interest cost	20	19
Benefits paid	(20)	(16)
Actuarial loss (gain)	(23)	83
Balance at end of year	480	491	395
Change in plan assets
Fair value of plan assets at beginning of year	435	386
Actual return (loss) on plan assets	4	34
Employer contributions	1	31
Fair value of plan assets at end of year	420	435	386
Accrued liability	(60)	(56)
Gulf Power [Member] | Other Postretirement Benefits [Member]
Change in benefit obligation
Benefit obligation at beginning of year	78	69
Service cost	1	1	1
Interest cost	3	3	3
Benefits paid	(4)	(4)
Plan amendments	4	(2)
Actuarial loss (gain)	(1)	11
Retiree drug subsidy	0	0
Balance at end of year	81	78	69
Change in plan assets
Fair value of plan assets at beginning of year	18	17
Actual return (loss) on plan assets	0	2
Employer contributions	3	3
Benefits paid, net of drug subsidy	(4)	(4)
Fair value of plan assets at end of year	17	18	17
Accrued liability	(64)	(60)
Mississippi Power [Member]
Change in benefit obligation
Service cost	13	10	11
Interest cost	21	20	18
Change in plan assets
Employer contributions	5	5	4
Mississippi Power [Member] | Pension Plans [Member]
Change in benefit obligation
Benefit obligation at beginning of year	513	409
Service cost	13	10
Interest cost	21	20
Benefits paid	(22)	(17)
Actuarial loss (gain)	(25)	91
Balance at end of year	500	513	409
Change in plan assets
Fair value of plan assets at beginning of year	446	387
Actual return (loss) on plan assets	4	40
Employer contributions	2	36
Fair value of plan assets at end of year	430	446	387
Accrued liability	(70)	(67)
Mississippi Power [Member] | Other Postretirement Benefits [Member]
Change in benefit obligation
Benefit obligation at beginning of year	96	81
Service cost	1	1	1
Interest cost	4	4	4
Benefits paid	(5)	(5)
Plan amendments	1	0
Actuarial loss (gain)	(1)	14
Retiree drug subsidy	1	1
Balance at end of year	97	96	81
Change in plan assets
Fair value of plan assets at beginning of year	24	23
Actual return (loss) on plan assets	0	2
Employer contributions	3	3
Benefits paid, net of drug subsidy	(4)	(4)
Fair value of plan assets at end of year	23	24	$ 23
Accrued liability	$ (74)	$ (72)