Leases - Other Information (Details) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($)
Cash paid for amounts included in the measurements of lease obligations:
Operating cash flows from operating leases	$ 211
Operating cash flows from finance leases	3
Financing cash flows from finance leases	24
ROU assets obtained in exchange for new operating lease obligations	76
ROU assets obtained in exchange for new finance lease obligations	$ 31
Weighted-average remaining lease term in years:
Operating leases	14 years 4 months 24 days
Finance leases	18 years 10 months 24 days
Weighted-average discount rate:
Operating leases	4.56%
Finance leases	5.07%
ALABAMA POWER CO
Cash paid for amounts included in the measurements of lease obligations:
Operating cash flows from operating leases	$ 50
Operating cash flows from finance leases	1
Financing cash flows from finance leases	0
ROU assets obtained in exchange for new operating lease obligations	7
ROU assets obtained in exchange for new finance lease obligations	$ 1
Weighted-average remaining lease term in years:
Operating leases	3 years 4 months 24 days
Finance leases	12 years 4 months 24 days
Weighted-average discount rate:
Operating leases	3.33%
Finance leases	3.65%
GEORGIA POWER CO
Cash paid for amounts included in the measurements of lease obligations:
Operating cash flows from operating leases	$ 130
Operating cash flows from finance leases	17
Financing cash flows from finance leases	4
ROU assets obtained in exchange for new operating lease obligations	18
ROU assets obtained in exchange for new finance lease obligations	$ 24
Weighted-average remaining lease term in years:
Operating leases	10 years 4 months 24 days
Finance leases	10 years 9 months 18 days
Weighted-average discount rate:
Operating leases	4.46%
Finance leases	10.74%
MISSISSIPPI POWER CO
Cash paid for amounts included in the measurements of lease obligations:
Operating cash flows from operating leases	$ 2
Operating cash flows from finance leases	0
Financing cash flows from finance leases	0
ROU assets obtained in exchange for new operating lease obligations	0
ROU assets obtained in exchange for new finance lease obligations	$ 0
Weighted-average remaining lease term in years:
Operating leases	6 years 9 months 18 days
Weighted-average discount rate:
Operating leases	3.98%
SOUTHERN POWER CO
Cash paid for amounts included in the measurements of lease obligations:
Operating cash flows from operating leases	$ 18
Operating cash flows from finance leases	0
Financing cash flows from finance leases	0
ROU assets obtained in exchange for new operating lease obligations	0
ROU assets obtained in exchange for new finance lease obligations	$ 0
Weighted-average remaining lease term in years:
Operating leases	33 years 2 months 12 days
Weighted-average discount rate:
Operating leases	5.68%
SOUTHERN Co GAS
Cash paid for amounts included in the measurements of lease obligations:
Operating cash flows from operating leases	$ 13
Operating cash flows from finance leases	0
Financing cash flows from finance leases	0
ROU assets obtained in exchange for new operating lease obligations	14
ROU assets obtained in exchange for new finance lease obligations	$ 0
Weighted-average remaining lease term in years:
Operating leases	9 years 7 months 6 days
Weighted-average discount rate:
Operating leases	3.73%