Commitments and Contingencies (Details-3) $ in Millions	Dec. 31, 2017 USD ($) pollutant state
Columbia Environmental Project, SCR [Member] | MGE [Member] | Minimum [Member]
Columbia
Share of projected SCR project cost	$ 19
Columbia Environmental Project, SCR [Member] | MGE [Member] | Maximum [Member]
Columbia
Share of projected SCR project cost	$ 24
Wisconsin Energy Efficiency and Renewables Act [Member]
Energy Efficiency and Renewables
Minimum electricity generated from renewable resources	10.00%
EPA [Member]
EPA's National Ambient Air Quality Standards
Number of pollutants monitored by the National Ambient Air Quality Standards | pollutant	6
EPA's Cross State Air Pollution Rule
Number of states incorporating 2008 ozone NAAQS attainment levels | state	22