Rate Matters (Details) - PSCW [Member] - MGE [Member] - USD ($)	1 Months Ended	4 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2017	Dec. 31, 2016	Jul. 31, 2017
Rate Proceedings [Abstract]
Authorized return on equity, percentage			9.80%
Approved equity capital structure, percentage			57.20%
Fuel Rules [Abstract]
Fuel rules, bandwidth			2.00%
Fuel rules, electric fuel deferred costs upper threshold			102.00%
Fuel rules, electric fuel deferred costs lower threshold			98.00%
Deferred fuel rules monitored costs			$ 3,300,000
Fuel Rules Credit ($0.00256/kWh), 2015/2016 [Member]
Fuel Rules [Abstract]
Electric fuel credit (per kWh)		$ 0.00256		$ 0.00256
Return of electric fuel credit, total		$ 2,600,000		$ 8,300,000
Fuel Rules Refund, 2015/2016 [Member]
Fuel Rules [Abstract]
Electric fuel credit ($)					$ 6,000,000
Return of electric fuel credit, total	$ 15,500,000
Electric Rate Proceeding [Member]
Rate Proceedings [Abstract]
Authorized rate increase (decrease), percentage			(0.80%)	0.00%
Authorized rate increase (decrease), amount			$ (3,300,000)	$ 0
Gas Rate Proceeding [Member]
Rate Proceedings [Abstract]
Authorized rate increase (decrease), percentage			1.90%	0.00%
Authorized rate increase (decrease), amount			$ 3,100,000	$ 0