Subsequent Event (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Aug. 31, 2017	May 31, 2017	May 31, 2016	Nov. 30, 2017	May 31, 2017	May 31, 2016
Subsequent Event [Line Items]
Special Charges		$ 4.7	$ 3.9		$ 8.3	$ 5.5
Maximum [Member] | MGE_Project [Member]
Subsequent Event [Line Items]
Special Charges		65.0
Expected cost savings special charges		40.0
Minimum [Member] | MGE_Project [Member]
Subsequent Event [Line Items]
Special Charges		55.0
Expected cost savings special charges		$ 30.0
Scenario, Forecast [Member] | MGE_Project [Member]
Subsequent Event [Line Items]
Special Charges	$ 2.0			$ 7.0