Rate Matters (Details) - PSCW [Member] - MGE [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Mar. 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	$ 1.5			$ 1.1
Fuel Rules Refund, 2015/2016 [Member]
Fuel Rules [Abstract]
Return of electric fuel credit, total			$ 6.2
Approved Future Rate Matters [Member]
Fuel Rules [Abstract]
Public Utilities Fuel Surcharge Amount		$ 0.5
Public Utilities Fuel Surcharge Percentage		0.13%
Electric Rate Proceeding [Member]
Rate Proceedings [Abstract]
Authorized rate increase (decrease), percentage			(0.80%)
Authorized rate increase (decrease), amount			$ (3.3)
Gas Rate Proceeding [Member]
Rate Proceedings [Abstract]
Authorized rate increase (decrease), percentage			1.90%
Authorized rate increase (decrease), amount			$ 3.1