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, 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%
Electric Fuel Monitored Costs, 2016 [Member]
Fuel Rules [Abstract]
Change in fuel rules monitored costs							$ (14,800,000)
Deferred fuel rules monitored costs	$ 4,100,000		$ 4,100,000
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	$ 6,300,000
Fuel Rules Refund, 2015/2016 [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			0.00%		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%