RETIREMENT BENEFITS - Changes in Projected Benefit Obligations and Fair Value of Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	$ 12,763	$ 13,808
Dispositions	(509)	(107)
Service cost	292	359	$ 293
Interest cost	492	464	455
Benefits paid	(596)	(618)
Actuarial (gain) loss	2,346	(1,143)
Balance at end of year	14,788	12,763	13,808
Change in plan assets
Fair value of plan assets at beginning of year	11,611	12,992
Dispositions	(509)	(107)
Actual return (loss) on plan assets	2,343	(711)
Employer contributions	1,208	55
Benefits paid	(596)	(618)
Fair value of plan assets at end of year	14,057	11,611	12,992
Accrued liability	(731)	(1,152)
Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	1,865	2,339
Dispositions	(69)	(18)
Service cost	18	24	24
Interest cost	69	75	79
Benefits paid	(126)	(129)
Actuarial (gain) loss	223	(432)
Retiree drug subsidy	5	6
Balance at end of year	1,985	1,865	2,339
Change in plan assets
Fair value of plan assets at beginning of year	928	1,053
Dispositions	(18)	(18)
Actual return (loss) on plan assets	189	(57)
Employer contributions	83	73
Benefits paid	(121)	(123)
Fair value of plan assets at end of year	1,061	928	1,053
Accrued liability	(924)	(937)
Alabama Power | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	2,816	2,998
Dispositions	0	0
Service cost	69	78	63
Interest cost	114	101	98
Benefits paid	(125)	(124)
Actuarial (gain) loss	530	(237)
Balance at end of year	3,404	2,816	2,998
Change in plan assets
Fair value of plan assets at beginning of year	2,575	2,836
Dispositions	0	0
Actual return (loss) on plan assets	524	(150)
Employer contributions	383	13
Benefits paid	(125)	(124)
Fair value of plan assets at end of year	3,357	2,575	2,836
Accrued liability	(47)	(241)
Alabama Power | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	403	517
Dispositions	0	0
Service cost	5	6	6
Interest cost	16	17	17
Benefits paid	(27)	(28)
Actuarial (gain) loss	63	(111)
Retiree drug subsidy	2	2
Balance at end of year	462	403	517
Change in plan assets
Fair value of plan assets at beginning of year	360	406
Dispositions	0	0
Actual return (loss) on plan assets	76	(25)
Employer contributions	2	5
Benefits paid	(25)	(26)
Fair value of plan assets at end of year	413	360	406
Accrued liability	(49)	(43)
Georgia Power | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	3,905	4,188
Dispositions	0	0
Service cost	74	87	74
Interest cost	156	139	138
Benefits paid	(194)	(191)
Actuarial (gain) loss	669	(318)
Balance at end of year	4,610	3,905	4,188
Change in plan assets
Fair value of plan assets at beginning of year	3,663	4,058
Dispositions	0	0
Actual return (loss) on plan assets	730	(218)
Employer contributions	243	14
Benefits paid	(194)	(191)
Fair value of plan assets at end of year	4,442	3,663	4,058
Accrued liability	(168)	(242)
Georgia Power | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	675	863
Dispositions	0	0
Service cost	5	6	7
Interest cost	26	28	29
Benefits paid	(47)	(47)
Actuarial (gain) loss	80	(178)
Retiree drug subsidy	3	3
Balance at end of year	742	675	863
Change in plan assets
Fair value of plan assets at beginning of year	344	386
Dispositions	0	0
Actual return (loss) on plan assets	68	(20)
Employer contributions	35	22
Benefits paid	(44)	(44)
Fair value of plan assets at end of year	403	344	386
Accrued liability	(339)	(331)
Mississippi Power | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	557	602
Dispositions	0	0
Service cost	12	17	15
Interest cost	22	20	20
Benefits paid	(26)	(24)
Actuarial (gain) loss	106	(58)
Balance at end of year	671	557	602
Change in plan assets
Fair value of plan assets at beginning of year	505	563
Dispositions	0	0
Actual return (loss) on plan assets	103	(37)
Employer contributions	59	3
Benefits paid	(26)	(24)
Fair value of plan assets at end of year	641	505	563
Accrued liability	(30)	(52)
Mississippi Power | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	81	97
Dispositions	0	0
Service cost	1	1	1
Interest cost	3	3	3
Benefits paid	(6)	(5)
Actuarial (gain) loss	8	(15)
Retiree drug subsidy	0	0
Balance at end of year	87	81	97
Change in plan assets
Fair value of plan assets at beginning of year	23	25
Dispositions	0	0
Actual return (loss) on plan assets	4	(1)
Employer contributions	5	4
Benefits paid	(6)	(5)
Fair value of plan assets at end of year	26	23	25
Accrued liability	(61)	(58)
Southern Power | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	123	139
Dispositions	0	(3)
Service cost	7	9
Interest cost	5	5
Benefits paid	(4)	(3)
Actuarial (gain) loss	54	(24)
Balance at end of year	185	123	139
Change in plan assets
Fair value of plan assets at beginning of year	123	138
Dispositions	0	(3)
Actual return (loss) on plan assets	43	(9)
Employer contributions	7	0
Benefits paid	(4)	(3)
Fair value of plan assets at end of year	169	123	138
Accrued liability	(16)	0
Southern Power | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	9	11
Dispositions	0	0
Service cost	1	1
Interest cost	0	0
Benefits paid	(1)	(1)
Actuarial (gain) loss	2	(2)
Retiree drug subsidy	0	0
Balance at end of year	11	9	11
Change in plan assets
Fair value of plan assets at beginning of year	0	0
Dispositions	0	0
Actual return (loss) on plan assets	0	0
Employer contributions	1	1
Benefits paid	(1)	(1)
Fair value of plan assets at end of year	0	0	0
Accrued liability	(11)	(9)
Southern Company Gas | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	907	1,184
Dispositions	0	(104)
Service cost	25	34	23
Interest cost	36	39	42
Benefits paid	(64)	(98)
Actuarial (gain) loss	163	(148)
Balance at end of year	1,067	907	1,184
Change in plan assets
Fair value of plan assets at beginning of year	798	1,068
Dispositions	0	(104)
Actual return (loss) on plan assets	172	(70)
Employer contributions	144	2
Benefits paid	(64)	(98)
Fair value of plan assets at end of year	1,050	798	1,068
Accrued liability	(17)	(109)
Southern Company Gas | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	244	310
Dispositions	0	(18)
Service cost	1	2	2
Interest cost	9	10	10
Benefits paid	(17)	(17)
Actuarial (gain) loss	13	(43)
Retiree drug subsidy	0	0
Balance at end of year	250	244	310
Change in plan assets
Fair value of plan assets at beginning of year	98	125
Dispositions	0	(18)
Actual return (loss) on plan assets	21	(5)
Employer contributions	13	13
Benefits paid	(17)	(17)
Fair value of plan assets at end of year	115	98	$ 125
Accrued liability	(135)	$ (146)
Qualified pension plan | Pension plans
Change in benefit obligation
Balance at end of year	14,055
Change in plan assets
Employer contributions	1,136
Qualified pension plan | Alabama Power | Pension plans
Change in benefit obligation
Balance at end of year	3,286
Change in plan assets
Employer contributions	362
Qualified pension plan | Georgia Power | Pension plans
Change in benefit obligation
Balance at end of year	4,480
Change in plan assets
Employer contributions	200
Qualified pension plan | Mississippi Power | Pension plans
Change in benefit obligation
Balance at end of year	639
Change in plan assets
Employer contributions	54
Qualified pension plan | Southern Power | Pension plans
Change in benefit obligation
Balance at end of year	159
Change in plan assets
Employer contributions	24
Qualified pension plan | Southern Company Gas | Pension plans
Change in benefit obligation
Balance at end of year	999
Change in plan assets
Employer contributions	145
Non-qualified pension plan | Pension plans
Change in benefit obligation
Balance at end of year	733
Non-qualified pension plan | Alabama Power | Pension plans
Change in benefit obligation
Balance at end of year	118
Non-qualified pension plan | Georgia Power | Pension plans
Change in benefit obligation
Balance at end of year	130
Non-qualified pension plan | Mississippi Power | Pension plans
Change in benefit obligation
Balance at end of year	31
Non-qualified pension plan | Southern Power | Pension plans
Change in benefit obligation
Balance at end of year	26
Non-qualified pension plan | Southern Company Gas | Pension plans
Change in benefit obligation
Balance at end of year	$ 68