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Retirement Benefits - Components of Net Periodic Benefit Cost and Estimated Future Benefit Payments (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension plans
Components of net periodic
Service cost			$ 359	$ 293	$ 262
Interest cost			464	455	422
Expected return on plan assets			(943)	(897)	(782)
Recognized net (gain) loss			213	162	150
Net amortization			4	12	14
Total change			97	25	66
Benefit payments:
Benefit Payments, 2019			623
Benefit Payments, 2020			645
Benefit Payments, 2021			664
Benefit Payments, 2022			687
Benefit Payments, 2023			711
Benefit Payments, 2024 to 2028			3,869
Other postretirement benefit plans
Components of net periodic
Service cost			24	24	22
Interest cost			75	79	76
Expected return on plan assets			(69)	(66)	(60)
Net amortization			21	20	21
Total change			51	57	59
Benefit payments:
Benefit Payments, 2019			136
Benefit Payments, 2020			136
Benefit Payments, 2021			136
Benefit Payments, 2022			137
Benefit Payments, 2023			137
Benefit Payments, 2024 to 2028			669
Subsidy receipts:
Subsidy Receipts, 2019			(7)
Subsidy Receipts, 2020			(7)
Subsidy Receipts, 2021			(8)
Subsidy Receipts, 2022			(8)
Subsidy Receipts, 2023			(8)
Subsidy Receipts, 2024 to 2028			(41)
Benefit Payments and Subsidy Receipts, Total
Benefit Payments and Subsidy Receipts, 2019			129
Benefit Payments and Subsidy Receipts, 2020			129
Benefit Payments and Subsidy Receipts, 2021			128
Benefit Payments and Subsidy Receipts, 2022			129
Benefit Payments and Subsidy Receipts, 2023			129
Benefit Payments and Subsidy Receipts, 2024 to 2028			628
Alabama Power \| Pension plans
Components of net periodic
Service cost			78	63	57
Interest cost			101	98	95
Expected return on plan assets			(207)	(196)	(184)
Recognized net (gain) loss			54	42	40
Net amortization			1	2	3
Total change			27	9	11
Benefit payments:
Benefit Payments, 2019			132
Benefit Payments, 2020			136
Benefit Payments, 2021			141
Benefit Payments, 2022			147
Benefit Payments, 2023			152
Benefit Payments, 2024 to 2028			832
Alabama Power \| Other postretirement benefit plans
Components of net periodic
Service cost			6	6	5
Interest cost			17	17	18
Expected return on plan assets			(26)	(25)	(25)
Net amortization			5	5	6
Total change			2	3	4
Benefit payments:
Benefit Payments, 2019			28
Benefit Payments, 2020			28
Benefit Payments, 2021			29
Benefit Payments, 2022			29
Benefit Payments, 2023			29
Benefit Payments, 2024 to 2028			146
Subsidy receipts:
Subsidy Receipts, 2019			(2)
Subsidy Receipts, 2020			(2)
Subsidy Receipts, 2021			(2)
Subsidy Receipts, 2022			(2)
Subsidy Receipts, 2023			(3)
Subsidy Receipts, 2024 to 2028			(13)
Benefit Payments and Subsidy Receipts, Total
Benefit Payments and Subsidy Receipts, 2019			26
Benefit Payments and Subsidy Receipts, 2020			26
Benefit Payments and Subsidy Receipts, 2021			27
Benefit Payments and Subsidy Receipts, 2022			27
Benefit Payments and Subsidy Receipts, 2023			26
Benefit Payments and Subsidy Receipts, 2024 to 2028			133
Georgia Power \| Pension plans
Components of net periodic
Service cost			87	74	70
Interest cost			139	138	136
Expected return on plan assets			(296)	(283)	(258)
Recognized net (gain) loss			69	57	55
Net amortization			2	3	5
Total change			1	(11)	8
Benefit payments:
Benefit Payments, 2019			201
Benefit Payments, 2020			206
Benefit Payments, 2021			209
Benefit Payments, 2022			215
Benefit Payments, 2023			221
Benefit Payments, 2024 to 2028			1,183
Georgia Power \| Other postretirement benefit plans
Components of net periodic
Service cost			6	7	6
Interest cost			28	29	30
Expected return on plan assets			(25)	(25)	(22)
Net amortization			10	9	10
Total change			19	20	24
Benefit payments:
Benefit Payments, 2019			51
Benefit Payments, 2020			50
Benefit Payments, 2021			50
Benefit Payments, 2022			50
Benefit Payments, 2023			49
Benefit Payments, 2024 to 2028			243
Subsidy receipts:
Subsidy Receipts, 2019			(3)
Subsidy Receipts, 2020			(3)
Subsidy Receipts, 2021			(3)
Subsidy Receipts, 2022			(3)
Subsidy Receipts, 2023			(4)
Subsidy Receipts, 2024 to 2028			(18)
Benefit Payments and Subsidy Receipts, Total
Benefit Payments and Subsidy Receipts, 2019			48
Benefit Payments and Subsidy Receipts, 2020			47
Benefit Payments and Subsidy Receipts, 2021			47
Benefit Payments and Subsidy Receipts, 2022			47
Benefit Payments and Subsidy Receipts, 2023			45
Benefit Payments and Subsidy Receipts, 2024 to 2028			225
Mississippi Power \| Pension plans
Components of net periodic
Service cost			17	15	13
Interest cost			20	20	19
Expected return on plan assets			(41)	(40)	(35)
Recognized net (gain) loss			10	7	7
Net amortization			0	1	1
Total change			6	3	5
Benefit payments:
Benefit Payments, 2019			28
Benefit Payments, 2020			28
Benefit Payments, 2021			29
Benefit Payments, 2022			29
Benefit Payments, 2023			30
Benefit Payments, 2024 to 2028			166
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)	(1)	(1)
Net amortization			1	1	1
Total change			3	4	$ 4
Benefit payments:
Benefit Payments, 2019			6
Benefit Payments, 2020			6
Benefit Payments, 2021			6
Benefit Payments, 2022			6
Benefit Payments, 2023			7
Benefit Payments, 2024 to 2028			30
Subsidy receipts:
Subsidy Receipts, 2019			0
Subsidy Receipts, 2020			0
Subsidy Receipts, 2021			0
Subsidy Receipts, 2022			(1)
Subsidy Receipts, 2023			(1)
Subsidy Receipts, 2024 to 2028			(2)
Benefit Payments and Subsidy Receipts, Total
Benefit Payments and Subsidy Receipts, 2019			6
Benefit Payments and Subsidy Receipts, 2020			6
Benefit Payments and Subsidy Receipts, 2021			6
Benefit Payments and Subsidy Receipts, 2022			5
Benefit Payments and Subsidy Receipts, 2023			6
Benefit Payments and Subsidy Receipts, 2024 to 2028			28
Southern Power \| Pension plans
Components of net periodic
Service cost			9	0
Interest cost			5	0
Expected return on plan assets			(10)
Recognized net (gain) loss			1
Net amortization			0
Total change			5
Benefit payments:
Benefit Payments, 2019			3
Benefit Payments, 2020			3
Benefit Payments, 2021			4
Benefit Payments, 2022			4
Benefit Payments, 2023			5
Benefit Payments, 2024 to 2028			27
Southern Power \| Other postretirement benefit plans
Components of net periodic
Service cost			1	0
Interest cost			0	0
Expected return on plan assets			0
Net amortization			0
Total change			1
Benefit payments:
Benefit Payments, 2019			0
Benefit Payments, 2020			0
Benefit Payments, 2021			0
Benefit Payments, 2022			1
Benefit Payments, 2023			1
Benefit Payments, 2024 to 2028			3
Subsidy receipts:
Subsidy Receipts, 2019			0
Subsidy Receipts, 2020			0
Subsidy Receipts, 2021			0
Subsidy Receipts, 2022			0
Subsidy Receipts, 2023			0
Subsidy Receipts, 2024 to 2028			0
Benefit Payments and Subsidy Receipts, Total
Benefit Payments and Subsidy Receipts, 2019			0
Benefit Payments and Subsidy Receipts, 2020			0
Benefit Payments and Subsidy Receipts, 2021			0
Benefit Payments and Subsidy Receipts, 2022			1
Benefit Payments and Subsidy Receipts, 2023			1
Benefit Payments and Subsidy Receipts, 2024 to 2028			3
Southern Company Gas \| Pension plans
Components of net periodic
Service cost	$ 15		34	23
Interest cost	20		39	42
Expected return on plan assets	(35)		(75)	(70)
Recognized net (gain) loss	14		12	18
Amortization of regulatory assets()	0		15	1
Net amortization	(1)		(2)	0
Total change	13		23	14
Benefit payments:
Benefit Payments, 2019			59
Benefit Payments, 2020			61
Benefit Payments, 2021			62
Benefit Payments, 2022			62
Benefit Payments, 2023			62
Benefit Payments, 2024 to 2028			313
Southern Company Gas \| Other postretirement benefit plans
Components of net periodic
Service cost	1		2	2
Interest cost	5		10	10
Expected return on plan assets	(3)		(7)	(7)
Amortization of regulatory assets()	2		6	0
Net amortization	0		0	(3)
Amortization of net gain (loss)	0		0	4
Total change	$ 5		11	$ 6
Benefit payments:
Benefit Payments, 2019			18
Benefit Payments, 2020			18
Benefit Payments, 2021			19
Benefit Payments, 2022			19
Benefit Payments, 2023			19
Benefit Payments, 2024 to 2028			90
Subsidy receipts:
Subsidy Receipts, 2019			0
Subsidy Receipts, 2020			0
Subsidy Receipts, 2021			0
Subsidy Receipts, 2022			0
Subsidy Receipts, 2023			0
Subsidy Receipts, 2024 to 2028			0
Benefit Payments and Subsidy Receipts, Total
Benefit Payments and Subsidy Receipts, 2019			18
Benefit Payments and Subsidy Receipts, 2020			18
Benefit Payments and Subsidy Receipts, 2021			19
Benefit Payments and Subsidy Receipts, 2022			19
Benefit Payments and Subsidy Receipts, 2023			19
Benefit Payments and Subsidy Receipts, 2024 to 2028			$ 90
Predecessor \| Southern Company Gas \| Pension plans
Components of net periodic
Service cost		$ 13
Interest cost		21
Expected return on plan assets		(33)
Recognized net (gain) loss		13
Amortization of regulatory assets()		0
Net amortization		(1)
Total change		13
Predecessor \| Southern Company Gas \| Other postretirement benefit plans
Components of net periodic
Service cost		1
Interest cost		5
Expected return on plan assets		(3)
Amortization of regulatory assets()		0
Net amortization		(1)
Amortization of net gain (loss)		2
Total change		$ 4