Rate Matters (Details) - PSCW [Member] - MGE [Member] - USD ($)	1 Months Ended	3 Months Ended	4 Months Ended	12 Months Ended
	Sep. 30, 2016	Mar. 31, 2017	Dec. 31, 2015	Dec. 31, 2016	Jul. 31, 2016	Mar. 31, 2016	Jan. 31, 2016	Aug. 31, 2015
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		$ 1,100,000				$ 0
Electric Fuel Monitored Costs, 2016 [Member]
Fuel Rules [Abstract]
Change in fuel rules monitored costs							$ (14,800,000)
Fuel Rules Credit ($0.00256/kWh), 2015/2016 [Member]
Fuel Rules [Abstract]
Electric fuel credit (per kWh)			$ 0.00256	$ 0.00256
Electric fuel credit ($)								$ 10,900,000
Return of electric fuel credit, total			$ 2,600,000	$ 8,300,000
Fuel Rules Refund, 2015/2016 [Member] | Electric Fuel Monitored Costs, 2016 [Member]
Fuel Rules [Abstract]
Electric fuel credit ($)					$ 15,700,000
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