Rate Matters (Details) - PSCW [Member] - MGE [Member] - USD ($) $ in Millions		9 Months Ended	12 Months Ended
	Aug. 31, 2018	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 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		$ 7.2				$ 4.2
Public Utilities Fuel Surcharge Amount					$ 0.5
Public Utilities Fuel Surcharge Percentage					0.10%
Fuel Rules Refund, 2015/2016 [Member]
Fuel Rules [Abstract]
Return of electric fuel credit, total						$ 6.2
Fuel Rules Refund, 2017 [Member]
Fuel Rules [Abstract]
Return of electric fuel credit, total	$ 4.2
Proposed Future Rate Matters [Member]
Rate Proceedings [Abstract]
Proposed return on equity, percentage			9.80%	9.80%
Proposed equity capital structure, percentage			56.10%	56.60%
Electric Rate Proceeding [Member]
Rate Proceedings [Abstract]
Authorized rate increase (decrease), percentage						(0.80%)
Authorized rate increase (decrease), amount						$ (3.3)
Electric Rate Proceeding [Member] | Proposed Future Rate Matters [Member]
Rate Proceedings [Abstract]
Proposed rate increase (decrease), percentage			0.00%	(1.94%)
Gas Rate Proceeding [Member]
Rate Proceedings [Abstract]
Authorized rate increase (decrease), percentage						1.90%
Authorized rate increase (decrease), amount						$ 3.1
Gas Rate Proceeding [Member] | Proposed Future Rate Matters [Member]
Rate Proceedings [Abstract]
Proposed rate increase (decrease), percentage			1.46%	1.06%