Regulatory Matters - Schedule of Cost Recovery Clauses (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Loss Contingencies [Line Items]
Natural gas cost under recovery	$ 485	$ 0
ALABAMA POWER CO | Other regulatory liabilities, current
Loss Contingencies [Line Items]
Rate CNP Compliance	1	28
Retail Energy Cost Recovery	0	18
ALABAMA POWER CO | Other regulatory liabilities, deferred
Loss Contingencies [Line Items]
Natural Disaster Reserve	36	77
ALABAMA POWER CO | Other regulatory assets, deferred
Loss Contingencies [Line Items]
Rate CNP PPA	64	58
Retail Energy Cost Recovery	54	0
GEORGIA POWER CO | Over recovered fuel clause revenues
Loss Contingencies [Line Items]
Fuel Cost Recovery	0	113
GEORGIA POWER CO | Other deferred charges and assets
Loss Contingencies [Line Items]
Fuel Cost Recovery	21	0
MISSISSIPPI POWER CO | Other regulatory liabilities, deferred
Loss Contingencies [Line Items]
Property Damage Reserve	0	4
MISSISSIPPI POWER CO | Other regulatory assets, deferred
Loss Contingencies [Line Items]
Ad Valorem Tax	45	41
Property Damage Reserve	5	0
MISSISSIPPI POWER CO | Over recovered regulatory clause liabilities
Loss Contingencies [Line Items]
Fuel Cost Recovery	9	24
MISSISSIPPI POWER CO | Other regulatory assets, current
Loss Contingencies [Line Items]
Ad Valorem Tax	12	11
SOUTHERN Co GAS
Loss Contingencies [Line Items]
Natural gas cost under recovery	485	0
SOUTHERN Co GAS | Other regulatory liabilities
Loss Contingencies [Line Items]
Natural gas cost under recovery	5	88
SOUTHERN Co GAS | Other regulatory assets, deferred
Loss Contingencies [Line Items]
Natural gas cost under recovery	119	0
SOUTHERN Co GAS | Other regulatory assets, current
Loss Contingencies [Line Items]
Natural gas cost under recovery	$ 485	$ 0