LEASES - Leases Not Yet Commenced (Details) $ in Millions	Dec. 31, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Longest lease term expiration	40 years
Estimated total obligations	$ 248
Alabama Power
Lessee, Lease, Description [Line Items]
Estimated total obligations	95
Mississippi Power
Lessee, Lease, Description [Line Items]
Estimated total obligations	23
Southern Power
Lessee, Lease, Description [Line Items]
Estimated total obligations	$ 87
Minimum
Lessee, Lease, Description [Line Items]
Longest lease term expiration	15 years
Maximum
Lessee, Lease, Description [Line Items]
Longest lease term expiration	40 years
Maximum | Alabama Power
Lessee, Lease, Description [Line Items]
Longest lease term expiration	28 years
Maximum | Mississippi Power
Lessee, Lease, Description [Line Items]
Longest lease term expiration	15 years
Maximum | Southern Power
Lessee, Lease, Description [Line Items]
Longest lease term expiration	40 years