Financing - Narrative (Details) $ in Millions	Mar. 31, 2021 USD ($)
Debt Instrument [Line Items]
Variable rate pollution control revenue bonds outstanding	$ 1,400
ALABAMA POWER CO
Debt Instrument [Line Items]
Variable rate pollution control revenue bonds outstanding	854
GEORGIA POWER CO
Debt Instrument [Line Items]
Variable rate pollution control revenue bonds outstanding	550
Fixed rate pollution control revenue bonds outstanding	174
MISSISSIPPI POWER CO
Debt Instrument [Line Items]
Variable rate pollution control revenue bonds outstanding	34
Fixed rate pollution control revenue bonds outstanding	$ 50