Contingencies and Regulatory Matters - Kemper County Energy Facility (Details) - MISSISSIPPI POWER CO - USD ($) $ in Millions		3 Months Ended	6 Months Ended		12 Months Ended
	Aug. 06, 2018	Jun. 30, 2018	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2017
Loss Contingencies [Line Items]
Accrued plant closure costs		$ 42		$ 42			$ 35
Kemper County Energy Facility
Loss Contingencies [Line Items]
Public utilities, property, plant and equipment, amount of loss (recovery) on plant abandonment		0		45
Public utilities, property, plant and equipment, amount of loss (recovery) on plant abandonment after tax				33
Accrued plant closure costs		$ 25		$ 25
Scenario, Forecast | Kemper County Energy Facility
Loss Contingencies [Line Items]
Production costs, period cost			$ 4		$ 4	$ 7
Subsequent Event | Reserve Margin Plan | Plant Watson, Unit 4
Loss Contingencies [Line Items]
Accelerated retirement period	2 years
Subsequent Event | Reserve Margin Plan | Plant Watson, Unit 5
Loss Contingencies [Line Items]
Accelerated retirement period	7 years
Subsequent Event | Reserve Margin Plan | Plant Greene County Units 1 and 2
Loss Contingencies [Line Items]
Accelerated retirement period	4 years