Retirement Benefits - Components of Net Periodic Benefit Cost and Estimated Future Benefit Payments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Other Postretirement Benefits [Member]
Components of net periodic
Service cost	$ 23	$ 21	$ 24
Interest cost	78	79	74
Expected return on plan assets	(58)	(59)	(56)
Net amortization	21	6	21
Net periodic benefit cost	64	47	63
Benefit Payments
Benefit Payments, 2016	123
Benefit Payments, 2017	128
Benefit Payments, 2018	133
Benefit Payments, 2019	137
Benefit Payments, 2020	139
Benefit Payments, 2021 to 2025	711
Subsidy Receipts
Subsidy Receipts, 2016	(9)
Subsidy Receipts, 2017	(10)
Subsidy Receipts, 2018	(11)
Subsidy Receipts, 2019	(12)
Subsidy Receipts, 2020	(12)
Subsidy Receipts, 2021 to 2025	(65)
Benefit Payments and Subsidy Receipts, Total
Benefit Payments and Subsidy Receipts, 2016	114
Benefit Payments and Subsidy Receipts, 2017	118
Benefit Payments and Subsidy Receipts, 2018	122
Benefit Payments and Subsidy Receipts, 2019	125
Benefit Payments and Subsidy Receipts, 2020	127
Benefit Payments and Subsidy Receipts, 2021 to 2025	646
Pension Plans [Member]
Components of net periodic
Service cost	257	213	232
Interest cost	445	435	389
Expected return on plan assets	(724)	(645)	(603)
Recognized net (gain) loss	215	110	200
Net amortization	25	26	27
Net periodic benefit cost	218	139	245
Benefit Payments
Benefit Payments, 2016	450
Benefit Payments, 2017	478
Benefit Payments, 2018	501
Benefit Payments, 2019	527
Benefit Payments, 2020	554
Benefit Payments, 2021 to 2025	3,141
Alabama Power [Member] | Other Postretirement Benefits [Member]
Components of net periodic
Service cost	6	5	6
Interest cost	20	20	19
Expected return on plan assets	(26)	(25)	(23)
Net amortization	5	4	5
Net periodic benefit cost	5	4	7
Benefit Payments
Benefit Payments, 2016	33
Benefit Payments, 2017	34
Benefit Payments, 2018	34
Benefit Payments, 2019	35
Benefit Payments, 2020	36
Benefit Payments, 2021 to 2025	184
Subsidy Receipts
Subsidy Receipts, 2016	(3)
Subsidy Receipts, 2017	(3)
Subsidy Receipts, 2018	(3)
Subsidy Receipts, 2019	(4)
Subsidy Receipts, 2020	(4)
Subsidy Receipts, 2021 to 2025	(20)
Benefit Payments and Subsidy Receipts, Total
Benefit Payments and Subsidy Receipts, 2016	30
Benefit Payments and Subsidy Receipts, 2017	31
Benefit Payments and Subsidy Receipts, 2018	31
Benefit Payments and Subsidy Receipts, 2019	31
Benefit Payments and Subsidy Receipts, 2020	32
Benefit Payments and Subsidy Receipts, 2021 to 2025	164
Alabama Power [Member] | Pension Plans [Member]
Components of net periodic
Service cost	59	48	52
Interest cost	106	103	93
Expected return on plan assets	(178)	(168)	(157)
Recognized net (gain) loss	55	31	52
Net amortization	6	7	7
Net periodic benefit cost	48	21	47
Benefit Payments
Benefit Payments, 2016	114
Benefit Payments, 2017	119
Benefit Payments, 2018	124
Benefit Payments, 2019	129
Benefit Payments, 2020	134
Benefit Payments, 2021 to 2025	740
Georgia Power [Member] | Other Postretirement Benefits [Member]
Components of net periodic
Service cost	7	6	7
Interest cost	34	34	31
Expected return on plan assets	(24)	(25)	(24)
Net amortization	11	2	12
Net periodic benefit cost	28	17	26
Benefit Payments
Benefit Payments, 2016	53
Benefit Payments, 2017	55
Benefit Payments, 2018	58
Benefit Payments, 2019	59
Benefit Payments, 2020	60
Benefit Payments, 2021 to 2025	305
Subsidy Receipts
Subsidy Receipts, 2016	(4)
Subsidy Receipts, 2017	(4)
Subsidy Receipts, 2018	(5)
Subsidy Receipts, 2019	(5)
Subsidy Receipts, 2020	(5)
Subsidy Receipts, 2021 to 2025	(28)
Benefit Payments and Subsidy Receipts, Total
Benefit Payments and Subsidy Receipts, 2016	49
Benefit Payments and Subsidy Receipts, 2017	51
Benefit Payments and Subsidy Receipts, 2018	53
Benefit Payments and Subsidy Receipts, 2019	54
Benefit Payments and Subsidy Receipts, 2020	55
Benefit Payments and Subsidy Receipts, 2021 to 2025	277
Georgia Power [Member] | Pension Plans [Member]
Components of net periodic
Service cost	73	62	69
Interest cost	154	153	138
Expected return on plan assets	(251)	(228)	(212)
Recognized net (gain) loss	76	41	74
Net amortization	9	10	10
Net periodic benefit cost	61	38	79
Benefit Payments
Benefit Payments, 2016	168
Benefit Payments, 2017	176
Benefit Payments, 2018	183
Benefit Payments, 2019	189
Benefit Payments, 2020	197
Benefit Payments, 2021 to 2025	1,085
Gulf Power [Member]
Components of net periodic
Service cost	12	10	11
Interest cost	20	19	17
Expected return on plan assets	(32)	(28)	(26)
Recognized net (gain) loss	9	5	9
Net amortization	1	1	1
Net periodic benefit cost	10	7	12
Gulf Power [Member] | Other Postretirement Benefits [Member]
Components of net periodic
Service cost	1	1	1
Interest cost	3	3	3
Expected return on plan assets	(1)	(1)	(1)
Net amortization	0	0	0
Net periodic benefit cost	3	3	3
Benefit Payments
Benefit Payments, 2016	5
Benefit Payments, 2017	5
Benefit Payments, 2018	6
Benefit Payments, 2019	6
Benefit Payments, 2020	6
Benefit Payments, 2021 to 2025	29
Subsidy Receipts
Subsidy Receipts, 2016	0
Subsidy Receipts, 2017	0
Subsidy Receipts, 2018	0
Subsidy Receipts, 2019	(1)
Subsidy Receipts, 2020	(1)
Subsidy Receipts, 2021 to 2025	(3)
Benefit Payments and Subsidy Receipts, Total
Benefit Payments and Subsidy Receipts, 2016	5
Benefit Payments and Subsidy Receipts, 2017	5
Benefit Payments and Subsidy Receipts, 2018	6
Benefit Payments and Subsidy Receipts, 2019	5
Benefit Payments and Subsidy Receipts, 2020	5
Benefit Payments and Subsidy Receipts, 2021 to 2025	26
Gulf Power [Member] | Pension Plans [Member]
Components of net periodic
Service cost	12	10
Interest cost	20	19
Benefit Payments
Benefit Payments, 2016	19
Benefit Payments, 2017	20
Benefit Payments, 2018	21
Benefit Payments, 2019	22
Benefit Payments, 2020	24
Benefit Payments, 2021 to 2025	139
Mississippi Power [Member]
Components of net periodic
Service cost	13	10	11
Interest cost	21	20	18
Expected return on plan assets	(33)	(29)	(27)
Recognized net (gain) loss	10	5	10
Net amortization	1	1	1
Net periodic benefit cost	12	7	13
Mississippi Power [Member] | Other Postretirement Benefits [Member]
Components of net periodic
Service cost	1	1	1
Interest cost	4	4	4
Expected return on plan assets	(2)	(2)	(1)
Net amortization	1	0	0
Net periodic benefit cost	4	3	$ 4
Benefit Payments
Benefit Payments, 2016	6
Benefit Payments, 2017	6
Benefit Payments, 2018	6
Benefit Payments, 2019	7
Benefit Payments, 2020	7
Benefit Payments, 2021 to 2025	36
Subsidy Receipts
Subsidy Receipts, 2016	0
Subsidy Receipts, 2017	(1)
Subsidy Receipts, 2018	(1)
Subsidy Receipts, 2019	(1)
Subsidy Receipts, 2020	(1)
Subsidy Receipts, 2021 to 2025	(2)
Benefit Payments and Subsidy Receipts, Total
Benefit Payments and Subsidy Receipts, 2016	6
Benefit Payments and Subsidy Receipts, 2017	5
Benefit Payments and Subsidy Receipts, 2018	5
Benefit Payments and Subsidy Receipts, 2019	6
Benefit Payments and Subsidy Receipts, 2020	6
Benefit Payments and Subsidy Receipts, 2021 to 2025	34
Mississippi Power [Member] | Pension Plans [Member]
Components of net periodic
Service cost	13	10
Interest cost	21	$ 20
Benefit Payments
Benefit Payments, 2016	20
Benefit Payments, 2017	21
Benefit Payments, 2018	22
Benefit Payments, 2019	24
Benefit Payments, 2020	25
Benefit Payments, 2021 to 2025	$ 146