INCOME TAXES - NOL Carryforwards (Details) $ in Millions	Dec. 31, 2019 USD ($)
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	$ 306
Mississippi Power | Mississippi
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	201
Southern Power
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	54
Southern Power | Oklahoma
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	39
Southern Power | Florida
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	11
Southern Power | South Carolina
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	2
Southern Power | Other states
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	2
Other Subsidiaries | Other states
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	18
Other Subsidiaries | Georgia
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	7
Other Subsidiaries | New York
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	11
Other Subsidiaries | New York City
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	15
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	7,230
State and Local Jurisdiction | Mississippi Power | Mississippi
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	5,099
State and Local Jurisdiction | Southern Power
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	1,165
State and Local Jurisdiction | Southern Power | Oklahoma
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	830
State and Local Jurisdiction | Southern Power | Florida
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	258
State and Local Jurisdiction | Southern Power | South Carolina
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	56
State and Local Jurisdiction | Southern Power | Other states
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	21
State and Local Jurisdiction | Other Subsidiaries | Other states
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	368
State and Local Jurisdiction | Other Subsidiaries | Georgia
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	171
State and Local Jurisdiction | Other Subsidiaries | New York
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	220
State and Local Jurisdiction | Other Subsidiaries | New York City
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	$ 207