RETIREMENT BENEFITS - Components of Net Periodic Benefit Cost and Estimated Future Benefit Payments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Pension plans
Components of net periodic
Service cost	$ 434	$ 376	$ 292
Interest cost	346	432	492
Expected return on plan assets	(1,191)	(1,100)	(885)
Recognized net loss	314	269	120
Net amortization	1	1	2
Prior service cost	0	0	0
Total change	(96)	(22)	21
Benefit payments:
2022	690
2023	714
2024	736
2025	759
2026	780
2027 to 2031	4,138
Other postretirement benefit plans
Components of net periodic
Service cost	24	22	18
Interest cost	35	54	69
Expected return on plan assets	(76)	(72)	(65)
Prior service cost	2	1	0
Total change	(15)	5	22
Benefit payments:
2022	111
2023	110
2024	109
2025	112
2026	112
2027 to 2031	552
Alabama Power | Pension plans
Components of net periodic
Service cost	102	89	69
Interest cost	82	100	114
Expected return on plan assets	(287)	(264)	(206)
Recognized net loss	82	71	37
Net amortization	1	1	0
Prior service cost	0	0	0
Total change	(20)	(3)	14
Benefit payments:
2022	148
2023	155
2024	159
2025	165
2026	171
2027 to 2031	921
Alabama Power | Other postretirement benefit plans
Components of net periodic
Service cost	6	6	5
Interest cost	9	13	16
Expected return on plan assets	(30)	(29)	(26)
Prior service cost	0	0	4
Total change	(15)	(10)	(1)
Benefit payments:
2022	24
2023	24
2024	24
2025	25
2026	25
2027 to 2031	128
Georgia Power | Pension plans
Components of net periodic
Service cost	112	96	74
Interest cost	104	133	156
Expected return on plan assets	(375)	(347)	(292)
Recognized net loss	100	86	44
Net amortization	1	1	1
Prior service cost	0	0	0
Total change	(58)	(31)	(17)
Benefit payments:
2022	223
2023	229
2024	235
2025	240
2026	245
2027 to 2031	1,275
Georgia Power | Other postretirement benefit plans
Components of net periodic
Service cost	7	6	5
Interest cost	12	20	26
Expected return on plan assets	(26)	(26)	(25)
Prior service cost	2	2	1
Total change	(5)	2	7
Benefit payments:
2022	40
2023	39
2024	38
2025	40
2026	40
2027 to 2031	199
Mississippi Power | Pension plans
Components of net periodic
Service cost	18	15	12
Interest cost	16	20	22
Expected return on plan assets	(55)	(51)	(40)
Recognized net loss	15	13	6
Net amortization	0	0	0
Prior service cost	0	0	0
Total change	(6)	(3)	0
Benefit payments:
2022	31
2023	31
2024	32
2025	34
2026	35
2027 to 2031	187
Mississippi Power | Other postretirement benefit plans
Components of net periodic
Service cost	1	1	1
Interest cost	1	2	3
Expected return on plan assets	(1)	(1)	(2)
Prior service cost	0	0	0
Total change	1	2	2
Benefit payments:
2022	5
2023	5
2024	5
2025	5
2026	5
2027 to 2031	22
Southern Power | Pension plans
Components of net periodic
Service cost	10	8	7
Interest cost	5	6	5
Expected return on plan assets	(14)	(13)	(10)
Recognized net loss	3	2	1
Net amortization	0	0	0
Prior service cost	0	0	0
Total change	4	3	3
Benefit payments:
2022	6
2023	6
2024	6
2025	7
2026	7
2027 to 2031	42
Southern Power | Other postretirement benefit plans
Components of net periodic
Service cost	0	1	1
Interest cost	0	0	0
Expected return on plan assets	1	0	0
Prior service cost	0	0	0
Total change	1	1	1
Benefit payments:
2022	0
2023	0
2024	0
2025	1
2026	1
2027 to 2031	3
Southern Company Gas | Pension plans
Components of net periodic
Service cost	37	33	25
Interest cost	24	31	36
Expected return on plan assets	(86)	(75)	(60)
Recognized net loss	13	6	2
Net amortization	15	15	14
Prior service cost	(3)	(3)	(3)
Total change	0	7	14
Benefit payments:
2022	65
2023	65
2024	64
2025	64
2026	64
2027 to 2031	329
Southern Company Gas | Other postretirement benefit plans
Components of net periodic
Service cost	2	2	1
Interest cost	4	7	9
Expected return on plan assets	(10)	(10)	(7)
Prior service cost	6	6	6
Total change	2	$ 5	$ 9
Benefit payments:
2022	17
2023	17
2024	18
2025	17
2026	17
2027 to 2031	$ 75