Contingencies and Regulatory Matters - Kemper County Energy Facility (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2017
Loss Contingencies [Line Items]
Estimated loss on Kemper IGCC	$ 44	$ 108
MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Estimated loss on Kemper IGCC	44	$ 108
Accrued plant closure costs	61					$ 35
Kemper County Energy Facility | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Public utilities, property, plant and equipment, amount of loss (recovery) on plant abandonment	44
Public utilities, property, plant and equipment, amount of loss (recovery) on plant abandonment after tax	33
Accrued plant closure costs	$ 50
Kemper County Energy Facility | Scenario, Forecast | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Production costs, period Cost			$ 4	$ 1	$ 4