LEASES - Other Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)
Cash paid for amounts included in the measurements of lease obligations:
Operating cash flows from operating leases	$ 323
Operating cash flows from finance leases	10
Financing cash flows from finance leases	32
ROU assets obtained in exchange for new operating lease obligations	118
ROU assets obtained in exchange for new finance lease obligations	$ 35
Weighted-average remaining lease term in years:
Operating leases	14 years 2 months 12 days
Finance leases	18 years 9 months 18 days
Weighted-average discount rate:
Operating leases	4.53%
PPA capacity expense	5.04%
Alabama Power
Cash paid for amounts included in the measurements of lease obligations:
Operating cash flows from operating leases	$ 54
Operating cash flows from finance leases	0
Financing cash flows from finance leases	1
ROU assets obtained in exchange for new operating lease obligations	7
ROU assets obtained in exchange for new finance lease obligations	$ 2
Weighted-average remaining lease term in years:
Operating leases	3 years 1 month 6 days
Finance leases	12 years 1 month 6 days
Weighted-average discount rate:
Operating leases	3.33%
PPA capacity expense	3.60%
Georgia Power
Cash paid for amounts included in the measurements of lease obligations:
Operating cash flows from operating leases	$ 210
Operating cash flows from finance leases	19
Financing cash flows from finance leases	13
ROU assets obtained in exchange for new operating lease obligations	21
ROU assets obtained in exchange for new finance lease obligations	$ 24
Weighted-average remaining lease term in years:
Operating leases	10 years 2 months 12 days
Finance leases	10 years 6 months
Weighted-average discount rate:
Operating leases	4.46%
PPA capacity expense	10.76%
Mississippi Power
Cash paid for amounts included in the measurements of lease obligations:
Operating cash flows from operating leases	$ 3
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	7 years
Weighted-average discount rate:
Operating leases	4.02%
Southern Power
Cash paid for amounts included in the measurements of lease obligations:
Operating cash flows from operating leases	$ 27
Operating cash flows from finance leases	0
Financing cash flows from finance leases	0
ROU assets obtained in exchange for new operating lease obligations	2
ROU assets obtained in exchange for new finance lease obligations	$ 0
Weighted-average remaining lease term in years:
Operating leases	32 years 9 months 18 days
Weighted-average discount rate:
Operating leases	5.66%
Southern Company Gas
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	19
ROU assets obtained in exchange for new finance lease obligations	$ 0
Weighted-average remaining lease term in years:
Operating leases	9 years 10 months 24 days
Weighted-average discount rate:
Operating leases	3.70%