Rate Matters (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2018	Oct. 31, 2019
Tax Cuts and Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Return of tax credit	$ 3,200,000		$ 8,200,000
PSCW [Member] | MGE [Member]
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	$ 500,000		9,500,000
Public Utilities Fuel Surcharge Amount			$ 500,000
Public Utilities Fuel Surcharge Percentage			0.13%
PSCW [Member] | MGE [Member] | Fuel Rules Refund, 2018 [Member] | Subsequent Event [Member]
Fuel Rules [Abstract]
Electric fuel credit ($)				$ 9,500,000
Electric Rate Proceeding [Member] | PSCW [Member] | MGE [Member]
Rate Proceedings [Abstract]
Authorized rate increase (decrease), percentage	(2.24%)	0.00%
Authorized rate increase (decrease), amount	$ (9,200,000)
Gas Rate Proceeding [Member] | PSCW [Member] | MGE [Member]
Rate Proceedings [Abstract]
Authorized rate increase (decrease), percentage	1.06%	1.46%
Authorized rate increase (decrease), amount	$ 1,700,000	$ 2,400,000