Leases - Leases Not Yet Commenced (Details) $ in Millions	Sep. 30, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Estimated total obligations	$ 159
ALABAMA POWER CO
Lessee, Lease, Description [Line Items]
Estimated total obligations	95
SOUTHERN POWER CO
Lessee, Lease, Description [Line Items]
Estimated total obligations	$ 64
Minimum
Lessee, Lease, Description [Line Items]
Longest lease term expiration	28 years
Maximum
Lessee, Lease, Description [Line Items]
Longest lease term expiration	31 years
Maximum | ALABAMA POWER CO
Lessee, Lease, Description [Line Items]
Longest lease term expiration	28 years
Maximum | SOUTHERN POWER CO
Lessee, Lease, Description [Line Items]
Longest lease term expiration	31 years