Income Taxes - NOL Carryforwards (Details) $ in Millions	Dec. 31, 2018 USD ($)
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	$ 312
Mississippi Power | Mississippi
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	200
Southern Power
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	56
Southern Power | Oklahoma
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	40
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	3
Other Subsidiaries | Other states
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	14
Other Subsidiaries | Georgia
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	16
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	7,343
State and Local Jurisdiction | Mississippi Power | Mississippi
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	5,062
State and Local Jurisdiction | Southern Power
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	1,214
State and Local Jurisdiction | Southern Power | Oklahoma
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	846
State and Local Jurisdiction | Southern Power | Florida
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	264
State and Local Jurisdiction | Southern Power | South Carolina
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	62
State and Local Jurisdiction | Southern Power | Other states
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	42
State and Local Jurisdiction | Other Subsidiaries | Other states
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	278
State and Local Jurisdiction | Other Subsidiaries | Georgia
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	358
State and Local Jurisdiction | Other Subsidiaries | New York
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	223
State and Local Jurisdiction | Other Subsidiaries | New York City
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	$ 208