Income Taxes - NOL Carryforwards (Details) $ in Millions	Dec. 31, 2017 USD ($)
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	$ 2,300
Approximate Net State Income Tax Benefit	249
Mississippi
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	114
Oklahoma
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	47
Georgia
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	23
New York
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	13
New York City
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	15
Florida
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	13
Other states
Operating Loss Carryforwards [Line Items]
Approximate Net State Income Tax Benefit	24
SOUTHERN POWER CO
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	1,332
Approximate Net State Income Tax Benefit	61
SOUTHERN POWER CO | Oklahoma
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	978
Approximate Net State Income Tax Benefit	46
SOUTHERN POWER CO | Florida
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	283
Approximate Net State Income Tax Benefit	12
SOUTHERN POWER CO | South Carolina
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	48
Approximate Net State Income Tax Benefit	2
SOUTHERN POWER CO | Other states
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	23
Approximate Net State Income Tax Benefit	1
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	5,607
State and Local Jurisdiction | Mississippi
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	2,890
State and Local Jurisdiction | Oklahoma
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	986
State and Local Jurisdiction | Georgia
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	524
State and Local Jurisdiction | New York
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	229
State and Local Jurisdiction | New York City
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	209
State and Local Jurisdiction | Florida
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	304
State and Local Jurisdiction | Other states
Operating Loss Carryforwards [Line Items]
Approximate NOL Carryforwards	$ 465