Regulatory Matters - MPC FERC Regulatory Matters (Details) - Mississippi Power - USD ($) $ in Millions				1 Months Ended					12 Months Ended
	Jan. 01, 2019	Feb. 06, 2018	May 01, 2016	Jan. 31, 2018	Sep. 30, 2017	May 31, 2017	Aug. 31, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2018
Schedule of Regulatory Assets and Liabilities [Line Items]
Increase in base rate under cost based electric tariff			$ 7.0
AFUDC cost								$ 22.0
Period of notice to cancel counterparty					10 years
Period of notice required for contract termination					1 year
Public utilities, approved rate increase (decrease), amount						$ 18.0	$ 18.0		$ 22.0
Kemper IGCC
Schedule of Regulatory Assets and Liabilities [Line Items]
Period of amortization of regulatory assets								36 months
Public utilities, approved rate increase (decrease), amount		$ 99.3
MRA Revenue
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease), amount				$ 11.0
Over recovered fuel cost										$ 6.0
MB Revenue
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease), amount				$ 1.0
Subsequent Event | MRA Revenue
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease), amount	$ (16.0)