Rate Matters (Details) - PSCW [Member] - MGE [Member] - USD ($)	3 Months Ended	4 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2015	Jan. 31, 2016
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	$ 3,600,000
Fuel Rules Credit, 2015/2016 [Member]
Fuel Rules [Abstract]
Electric fuel credit (per kWh)	0.00256	$ 0.00256
Fuel rules, deferred electric cost	7,600,000	9,500,000		$ 9,500,000
Return of electric fuel credit, total	$ 1,900,000	$ 2,600,000
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%