INCOME TAXES - NOL Carryforwards (Details) $ in Millions	Dec. 31, 2020 USD ($)
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit of NOL Carryforwards	$ 299
Mississippi Power | Mississippi
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit of NOL Carryforwards	200
Southern Power
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit of NOL Carryforwards	53
Southern Power | Oklahoma
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit of NOL Carryforwards	39
Southern Power | Florida
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit of NOL Carryforwards	11
Southern Power | South Carolina
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit of NOL Carryforwards	2
Southern Power | Other states
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit of NOL Carryforwards	1
Other Subsidiaries | Other states
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit of NOL Carryforwards	21
Other Subsidiaries | New York
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit of NOL Carryforwards	11
Other Subsidiaries | New York City
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit of NOL Carryforwards	$ 14