Regulatory Matters - Kemper County Energy Facility (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended	48 Months Ended
	Dec. 31, 2018	Dec. 31, 2019	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2023
Loss Contingencies [Line Items]
Liabilities incurred				$ 35
MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Liabilities incurred				1
MISSISSIPPI POWER CO | Kemper IGCC
Loss Contingencies [Line Items]
Pre-tax charge to income			$ 4	10
Asset retirement obligation, liabilities incurred, net of tax			3	7
Asset retirement obligation, expected cost			$ 10	$ 10
Government grants received	$ 387
MISSISSIPPI POWER CO | Kemper IGCC | Scenario, Forecast
Loss Contingencies [Line Items]
Liabilities incurred		$ 3
MISSISSIPPI POWER CO | Kemper IGCC | Minimum | Scenario, Forecast
Loss Contingencies [Line Items]
Liabilities incurred					$ 2
MISSISSIPPI POWER CO | Kemper IGCC | Maximum | Scenario, Forecast
Loss Contingencies [Line Items]
Liabilities incurred					$ 7