Rate Matters (Details) - PSCW [Member] - MGE [Member] - USD ($)	4 Months Ended	6 Months Ended	12 Months Ended
	Dec. 31, 2015	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2015	Jul. 31, 2016	Jan. 31, 2016	Aug. 31, 2015
Rate Proceedings [Abstract]
Authorized return on equity, percentage				10.20%
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		$ 14,800,000
Electric Fuel Monitored Costs, 2016 [Member]
Fuel Rules [Abstract]
Change in fuel rules monitored costs						$ (14,800,000)
Deferred fuel rules monitored costs		2,300,000
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	$ 3,900,000
Fuel Rules Refund, 2015/2016 [Member] | Subsequent Event [Member]
Fuel Rules [Abstract]
Electric fuel credit ($)					$ 15,700,000
Electric Rate Proceeding [Member]
Rate Proceedings [Abstract]
Authorized rate increase (decrease), percentage		0.00%		3.80%
Authorized rate increase (decrease), amount		$ 0		$ 15,400,000
Electric Rate Proceeding [Member] | Proposed Future Rate Matters [Member]
Rate Proceedings [Abstract]
Proposed rate increase (decrease), percentage			1.70%
Gas Rate Proceeding [Member]
Rate Proceedings [Abstract]
Authorized rate increase (decrease), percentage		0.00%		(2.00%)
Authorized rate increase (decrease), amount		$ 0		$ (3,800,000)
Gas Rate Proceeding [Member] | Proposed Future Rate Matters [Member]
Rate Proceedings [Abstract]
Proposed rate increase (decrease), percentage			3.70%