Leases - Major Categories of Lease Obligations (Details) $ in Millions	Sep. 30, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Lease obligations	$ 2,063
Electric generating units
Lessee, Lease, Description [Line Items]
Lease obligations	988
Real estate/land
Lessee, Lease, Description [Line Items]
Lease obligations	785
Communication towers
Lessee, Lease, Description [Line Items]
Lease obligations	150
Railcars
Lessee, Lease, Description [Line Items]
Lease obligations	51
Other
Lessee, Lease, Description [Line Items]
Lease obligations	89
ALABAMA POWER CO
Lessee, Lease, Description [Line Items]
Lease obligations	163
ALABAMA POWER CO | Electric generating units
Lessee, Lease, Description [Line Items]
Lease obligations	125
ALABAMA POWER CO | Real estate/land
Lessee, Lease, Description [Line Items]
Lease obligations	4
ALABAMA POWER CO | Communication towers
Lessee, Lease, Description [Line Items]
Lease obligations	2
ALABAMA POWER CO | Railcars
Lessee, Lease, Description [Line Items]
Lease obligations	23
ALABAMA POWER CO | Other
Lessee, Lease, Description [Line Items]
Lease obligations	9
GEORGIA POWER CO
Lessee, Lease, Description [Line Items]
Lease obligations	1,587
GEORGIA POWER CO | Electric generating units
Lessee, Lease, Description [Line Items]
Lease obligations	1,488
GEORGIA POWER CO | Real estate/land
Lessee, Lease, Description [Line Items]
Lease obligations	57
GEORGIA POWER CO | Communication towers
Lessee, Lease, Description [Line Items]
Lease obligations	3
GEORGIA POWER CO | Railcars
Lessee, Lease, Description [Line Items]
Lease obligations	25
GEORGIA POWER CO | Other
Lessee, Lease, Description [Line Items]
Lease obligations	14
MISSISSIPPI POWER CO
Lessee, Lease, Description [Line Items]
Lease obligations	7
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	2
SOUTHERN POWER CO
Lessee, Lease, Description [Line Items]
Lease obligations	395
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	395
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	91
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	77
SOUTHERN Co GAS | Communication towers
Lessee, Lease, Description [Line Items]
Lease obligations	13
SOUTHERN Co GAS | Railcars
Lessee, Lease, Description [Line Items]
Lease obligations	0
SOUTHERN Co GAS | Other
Lessee, Lease, Description [Line Items]
Lease obligations	$ 1