Leases - Major Categories of Lease Obligations (Details) $ in Millions	Mar. 31, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Lease obligations	$ 2,236
Electric generating units
Lessee, Lease, Description [Line Items]
Lease obligations	1,094
Real estate/land
Lessee, Lease, Description [Line Items]
Lease obligations	803
Communication towers
Lessee, Lease, Description [Line Items]
Lease obligations	131
Railcars
Lessee, Lease, Description [Line Items]
Lease obligations	55
Other
Lessee, Lease, Description [Line Items]
Lease obligations	153
ALABAMA POWER CO
Lessee, Lease, Description [Line Items]
Lease obligations	198
ALABAMA POWER CO | Electric generating units
Lessee, Lease, Description [Line Items]
Lease obligations	159
ALABAMA POWER CO | Real estate/land
Lessee, Lease, Description [Line Items]
Lease obligations	3
ALABAMA POWER CO | Communication towers
Lessee, Lease, Description [Line Items]
Lease obligations	1
ALABAMA POWER CO | Railcars
Lessee, Lease, Description [Line Items]
Lease obligations	25
ALABAMA POWER CO | Other
Lessee, Lease, Description [Line Items]
Lease obligations	10
GEORGIA POWER CO
Lessee, Lease, Description [Line Items]
Lease obligations	1,714
GEORGIA POWER CO | Electric generating units
Lessee, Lease, Description [Line Items]
Lease obligations	1,606
GEORGIA POWER CO | Real estate/land
Lessee, Lease, Description [Line Items]
Lease obligations	63
GEORGIA POWER CO | Communication towers
Lessee, Lease, Description [Line Items]
Lease obligations	3
GEORGIA POWER CO | Railcars
Lessee, Lease, Description [Line Items]
Lease obligations	26
GEORGIA POWER CO | Other
Lessee, Lease, Description [Line Items]
Lease obligations	16
MISSISSIPPI POWER CO
Lessee, Lease, Description [Line Items]
Lease obligations	8
MISSISSIPPI POWER CO | Electric generating units
Lessee, Lease, Description [Line Items]
Lease obligations	0
MISSISSIPPI POWER CO | Real estate/land
Lessee, Lease, Description [Line Items]
Lease obligations	2
MISSISSIPPI POWER CO | Communication towers
Lessee, Lease, Description [Line Items]
Lease obligations	0
MISSISSIPPI POWER CO | Railcars
Lessee, Lease, Description [Line Items]
Lease obligations	3
MISSISSIPPI POWER CO | Other
Lessee, Lease, Description [Line Items]
Lease obligations	3
SOUTHERN POWER CO
Lessee, Lease, Description [Line Items]
Lease obligations	393
SOUTHERN POWER CO | Electric generating units
Lessee, Lease, Description [Line Items]
Lease obligations	0
SOUTHERN POWER CO | Real estate/land
Lessee, Lease, Description [Line Items]
Lease obligations	393
SOUTHERN POWER CO | Communication towers
Lessee, Lease, Description [Line Items]
Lease obligations	0
SOUTHERN POWER CO | Railcars
Lessee, Lease, Description [Line Items]
Lease obligations	0
SOUTHERN POWER CO | Other
Lessee, Lease, Description [Line Items]
Lease obligations	0
SOUTHERN Co GAS
Lessee, Lease, Description [Line Items]
Lease obligations	84
SOUTHERN Co GAS | Electric generating units
Lessee, Lease, Description [Line Items]
Lease obligations	0
SOUTHERN Co GAS | Real estate/land
Lessee, Lease, Description [Line Items]
Lease obligations	83
SOUTHERN Co GAS | Communication towers
Lessee, Lease, Description [Line Items]
Lease obligations	0
SOUTHERN Co GAS | Railcars
Lessee, Lease, Description [Line Items]
Lease obligations	0
SOUTHERN Co GAS | Other
Lessee, Lease, Description [Line Items]
Lease obligations	$ 1