Financing - Narrative (Details) - USD ($)	Jun. 30, 2021	May 31, 2021	Apr. 30, 2021
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 7,633,000,000
Line of credit expiring in 2022	805,000,000
Variable rate pollution control revenue bonds outstanding	1,400,000,000
ALABAMA POWER CO
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	1,228,000,000	$ 700,000,000	$ 800,000,000
Line of credit expiring in 2022	525,000,000
Variable rate pollution control revenue bonds outstanding	854,000,000
SOUTHERN Co GAS
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	1,750,000,000	1,500,000,000	$ 1,750,000,000
Line of credit expiring in 2022	250,000,000
MISSISSIPPI POWER CO
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	275,000,000	$ 150,000,000
Line of credit expiring in 2022	0
Variable rate pollution control revenue bonds outstanding	34,000,000
Fixed rate pollution control revenue bonds outstanding	50,000,000
GEORGIA POWER CO
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	1,750,000,000
Line of credit expiring in 2022	0
Variable rate pollution control revenue bonds outstanding	550,000,000
Fixed rate pollution control revenue bonds outstanding	$ 105,000,000