Regulatory Matters - Mississippi Power (Details) - USD ($) $ in Millions	Jan. 01, 2019	Oct. 24, 2019
MISSISSIPPI POWER CO | MRA Revenue
Loss Contingencies [Line Items]
Decrease in base rate under cost based electric tariff	$ 3.7
Subsequent Event | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Proportionate ownership share		50.00%
Total estimated cost		$ 66.0
Subsequent Event | MISSISSIPPI POWER CO | Ash Pond Closure
Loss Contingencies [Line Items]
Total estimated cost		17.0
Subsequent Event | Mississippi Power And Gulf Power
Loss Contingencies [Line Items]
Total estimated cost		$ 125.0