RETIREMENT BENEFITS - Components of Net Periodic Benefit Cost and Estimated Future Benefit Payments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension plans
Components of net periodic
Service cost	$ 292	$ 359	$ 293
Interest cost	492	464	455
Expected return on plan assets	(885)	(943)	(897)
Recognized net (gain) loss	120	213	162
Net amortization	2	4	12
Prior service cost	0	0
Total change	21	97	25
Benefit payments:
2020	628
2021	646
2022	671
2023	693
2024	715
2025 to 2029	3,868
Other postretirement benefit plans
Components of net periodic
Service cost	18	24	24
Interest cost	69	75	79
Expected return on plan assets	(65)	(69)	(66)
Prior service cost	0	21	20
Total change	22	51	57
Benefit payments:
2020	130
2021	129
2022	129
2023	130
2024	129
2025 to 2029	630
Subsidy receipts:
2020	(5)
2021	(6)
2022	(6)
2023	(6)
2024	(6)
2025 to 2029	(30)
Benefit Payments and Subsidy Receipts, Total:
2020	125
2021	123
2022	123
2023	124
2024	123
2025 to 2029	600
Alabama Power | Pension plans
Components of net periodic
Service cost	69	78	63
Interest cost	114	101	98
Expected return on plan assets	(206)	(207)	(196)
Recognized net (gain) loss	37	54	42
Net amortization	0	1	2
Prior service cost	0	0
Total change	14	27	9
Benefit payments:
2020	135
2021	141
2022	147
2023	153
2024	157
2025 to 2029	860
Alabama Power | Other postretirement benefit plans
Components of net periodic
Service cost	5	6	6
Interest cost	16	17	17
Expected return on plan assets	(26)	(26)	(25)
Prior service cost	4	5	5
Total change	(1)	2	3
Benefit payments:
2020	29
2021	29
2022	29
2023	29
2024	29
2025 to 2029	145
Subsidy receipts:
2020	(1)
2021	(2)
2022	(2)
2023	(2)
2024	(2)
2025 to 2029	(9)
Benefit Payments and Subsidy Receipts, Total:
2020	28
2021	27
2022	27
2023	27
2024	27
2025 to 2029	136
Georgia Power | Pension plans
Components of net periodic
Service cost	74	87	74
Interest cost	156	139	138
Expected return on plan assets	(292)	(296)	(283)
Recognized net (gain) loss	44	69	57
Net amortization	1	2	3
Prior service cost	0	0
Total change	(17)	1	(11)
Benefit payments:
2020	204
2021	208
2022	214
2023	220
2024	226
2025 to 2029	1,209
Georgia Power | Other postretirement benefit plans
Components of net periodic
Service cost	5	6	7
Interest cost	26	28	29
Expected return on plan assets	(25)	(25)	(25)
Prior service cost	1	10	9
Total change	7	19	20
Benefit payments:
2020	49
2021	49
2022	49
2023	49
2024	48
2025 to 2029	238
Subsidy receipts:
2020	(2)
2021	(2)
2022	(3)
2023	(3)
2024	(3)
2025 to 2029	(13)
Benefit Payments and Subsidy Receipts, Total:
2020	47
2021	47
2022	46
2023	46
2024	45
2025 to 2029	225
Mississippi Power | Pension plans
Components of net periodic
Service cost	12	17	15
Interest cost	22	20	20
Expected return on plan assets	(40)	(41)	(40)
Recognized net (gain) loss	6	10	7
Net amortization	0	0	1
Prior service cost	0	0
Total change	0	6	3
Benefit payments:
2020	27
2021	28
2022	30
2023	30
2024	32
2025 to 2029	174
Mississippi Power | Other postretirement benefit plans
Components of net periodic
Service cost	1	1	1
Interest cost	3	3	3
Expected return on plan assets	(2)	(2)	(1)
Prior service cost	0	1	1
Total change	2	3	4
Benefit payments:
2020	6
2021	6
2022	6
2023	6
2024	6
2025 to 2029	29
Subsidy receipts:
2020	0
2021	0
2022	0
2023	0
2024	(1)
2025 to 2029	(2)
Benefit Payments and Subsidy Receipts, Total:
2020	6
2021	6
2022	6
2023	6
2024	5
2025 to 2029	27
Southern Power | Pension plans
Components of net periodic
Service cost	7	9
Interest cost	5	5
Expected return on plan assets	(10)	(10)
Recognized net (gain) loss	1	1
Net amortization	0	0
Prior service cost	0	0
Total change	3	5
Benefit payments:
2020	5
2021	6
2022	6
2023	6
2024	7
2025 to 2029	36
Southern Power | Other postretirement benefit plans
Components of net periodic
Service cost	1	1
Interest cost	0	0
Expected return on plan assets	0	0
Prior service cost	0	0
Total change	1	1
Benefit payments:
2020	0
2021	0
2022	1
2023	1
2024	1
2025 to 2029	3
Subsidy receipts:
2020	0
2021	0
2022	0
2023	0
2024	0
2025 to 2029	0
Benefit Payments and Subsidy Receipts, Total:
2020	0
2021	0
2022	1
2023	1
2024	1
2025 to 2029	3
Southern Company Gas | Pension plans
Components of net periodic
Service cost	25	34	23
Interest cost	36	39	42
Expected return on plan assets	(60)	(75)	(70)
Recognized net (gain) loss	2	12	18
Net amortization	14	15	1
Prior service cost	(3)	(2)
Total change	14	23	14
Benefit payments:
2020	62
2021	62
2022	64
2023	62
2024	62
2025 to 2029	316
Southern Company Gas | Other postretirement benefit plans
Components of net periodic
Service cost	1	2	2
Interest cost	9	10	10
Expected return on plan assets	(7)	(7)	(7)
Prior service cost	6	6	1
Total change	9	$ 11	$ 6
Benefit payments:
2020	18
2021	18
2022	18
2023	19
2024	18
2025 to 2029	83
Subsidy receipts:
2020	0
2021	0
2022	0
2023	0
2024	0
2025 to 2029	0
Benefit Payments and Subsidy Receipts, Total:
2020	18
2021	18
2022	18
2023	19
2024	18
2025 to 2029	$ 83