LEASES - Lease Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Lessor, Lease, Description [Line Items]
Lease income - interest income on sales-type leases	$ 16	$ 9
Lease income - operating leases	208	273
Variable lease income	419	403
Total lease income	643	685
Alabama Power
Lessor, Lease, Description [Line Items]
Lease income - interest income on sales-type leases	0	0
Lease income - operating leases	45	24
Variable lease income	0	0
Total lease income	45	24
Georgia Power
Lessor, Lease, Description [Line Items]
Lease income - interest income on sales-type leases	0	0
Lease income - operating leases	58	71
Variable lease income	0	0
Total lease income	58	71
Mississippi Power
Lessor, Lease, Description [Line Items]
Lease income - interest income on sales-type leases	12	9
Lease income - operating leases	2	0
Variable lease income	0	0
Total lease income	14	9
Southern Power
Lessor, Lease, Description [Line Items]
Lease income - interest income on sales-type leases	0	0
Lease income - operating leases	87	160
Variable lease income	449	434
Total lease income	536	594
Southern Company Gas
Lessor, Lease, Description [Line Items]
Lease income - interest income on sales-type leases	0	0
Lease income - operating leases	35	35
Variable lease income	0	0
Total lease income	$ 35	$ 35