Rate Matters (Details) - PSCW [Member] - MGE [Member] - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jul. 31, 2018	Jun. 30, 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	$ 5.2						$ 1.8
Public Utilities Fuel Surcharge Amount				$ 0.5
Public Utilities Fuel Surcharge Percentage				0.13%
Fuel Rules Refund, 2015/2016 [Member]
Fuel Rules [Abstract]
Return of electric fuel credit, total					$ 6.2
Fuel Rules Refund, 2017 [Member] | Subsequent Event [Member]
Fuel Rules [Abstract]
Electric fuel credit ($)						$ 4.2
Proposed Future Rate Matters [Member] | Subsequent Event [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] | Subsequent Event [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] | Subsequent Event [Member]
Rate Proceedings [Abstract]
Proposed rate increase (decrease), percentage		1.46%	1.06%