Rate Matters (Details) - PSCW [Member] - MGE [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2018
Rate Proceedings [Abstract]
Authorized return on equity, percentage	9.80%	9.80%
Approved equity capital structure, percentage	56.60%	56.10%
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	$ 0.0		$ 9.5
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	(2.24%)	0.00%
Authorized rate increase (decrease), amount	$ (9.2)
Gas Rate Proceeding [Member]
Rate Proceedings [Abstract]
Authorized rate increase (decrease), percentage	1.06%	1.46%
Authorized rate increase (decrease), amount	$ 1.7	$ 2.4