Joint Plant Ownership (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2020 USD ($) MW	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets		$ 157,252	$ 145,746
Other Regulatory Asset [Member]
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets		235	340
Columbia Units [Member]
Jointly Owned Plants, Net Plant Ownership [Abstract]
Plant	[1]	289,597	291,997
Accumulated depreciation	[1]	(118,742)	(105,778)
Construction work in progress	[1]	$ 997	1,777
Columbia Units [Member] | MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, plant capacity (in MW) | MW	[1]	211
Jointly owned utility plant, ownership interest	[1]	19.00%
Jointly owned utility plant, fuel, operating, and maintenance expense	[1]	$ 27,127	32,604	$ 36,517
Elm Road Units [Member] | MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, fuel, operating, and maintenance expense	[2]	$ 17,259	19,661	17,555
Elm Road Units [Member] | MGE Power Elm Road [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, plant capacity (in MW) | MW	[2]	106
Jointly owned utility plant, ownership interest	[2]	8.33%
Jointly Owned Plants, Net Plant Ownership [Abstract]
Plant	[2]	$ 203,847	207,834
Accumulated depreciation	[2]	(36,964)	(37,071)
Construction work in progress	[2]	697	468
Forward Wind [Member]
Jointly Owned Plants, Net Plant Ownership [Abstract]
Plant	[3]	34,028	34,054
Accumulated depreciation	[3]	(14,092)	(13,413)
Construction work in progress	[3]	$ 0	45
Forward Wind [Member] | MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, plant capacity (in MW) | MW	[3]	18
Jointly owned utility plant, ownership interest	[3]	12.80%
Jointly owned utility plant, fuel, operating, and maintenance expense	[3]	$ 664	642
Forward Wind [Member] | MGE [Member] | Other Regulatory Asset [Member]
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets	[3],[4]			600
West Campus [Member] | UW [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, fuel, operating, and maintenance expense	[5]	$ 5,200	6,600	6,300
West Campus [Member] | MGE Power West Campus [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, plant capacity (in MW) | MW	[6]	157
Jointly owned utility plant, ownership interest	[6]	55.00%
Jointly Owned Plants, Net Plant Ownership [Abstract]
Plant	[6]	$ 115,657	113,259
Accumulated depreciation	[6]	(36,580)	(33,786)
Construction work in progress	[6]	677	309
Two Creeks [Member]
Jointly Owned Plants, Net Plant Ownership [Abstract]
Plant	[7]	67,577	0
Accumulated depreciation	[7]	(225)	0
Construction work in progress	[7]	$ 0	45,286
Two Creeks [Member] | MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, plant capacity (in MW) | MW	[7]	50
Jointly owned utility plant, ownership interest	[7]	33.00%
Jointly owned utility plant, fuel, operating, and maintenance expense	[7]	$ 118	0	0
Badger Hollow I and II [Member]
Jointly Owned Plants, Net Plant Ownership [Abstract]
Plant	[8]	0	0
Accumulated depreciation	[8]	0	0
Construction work in progress	[8]	$ 63,140	18,953
Badger Hollow I and II [Member] | MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, plant capacity (in MW) | MW	[8]	100
Jointly owned utility plant, ownership interest	[8]	33.00%
Jointly owned utility plant, fuel, operating, and maintenance expense	[8]	$ 0	$ 0	$ 0

[1] In February 2021, MGE and the other co-owners announced plans to retire Columbia, a two unit coal-fired generation facility located in Portage, Wisconsin. The co-owners intend to retire Unit 1 by the end of 2023 and Unit 2 by the end of 2024. Final timing and retirement dates for Units 1 and 2 are subject to PSCW and regional regulatory reviews, including identification and approval of energy and capacity resources to replace Columbia.

[2] Two coal-fired generating units in Oak Creek, Wisconsin.

[3] The Forward Wind Energy Center (Forward Wind) consists of 86 wind turbines located near Brownsville, Wisconsin.

[4] Amount was deferred as a regulatory asset in 2018. This amount was recovered in rates and recognized as operating and maintenance expense in 2019.

[5] Operating charges are allocated to the UW based on formulas contained in the operating agreement. Under the provisions of this arrangement, the UW is required to reimburse MGE for their allocated portion of fuel and operating expenses. For the years ended December 31, 2020, 2019, and 2018, the UW's allocated share of fuel and operating costs was $5.2 million, $6.6 million, and $6.3 million, respectively.

[6] MGE Power West Campus and the UW jointly own the West Campus Cogeneration Facility (WCCF) located on the UW campus in Madison, Wisconsin. The UW owns a controlling interest in the chilled-water and steam plants, which are used to meet the needs for air-conditioning and steam-heat capacity for the UW campus. MGE Power West Campus owns a controlling interest in the electric generation plant, which is leased and operated by MGE.

[7] The Two Creeks solar generation array is located in the Town of Two Creeks and the City of Two Rivers in Manitowoc and Kewaunee Counties, Wisconsin. Date of commercial operation of the solar array was November 2020.

[8] The Badger Hollow I and II solar farm is located in southwestern Wisconsin in Iowa County, near the villages of Montfort and Cobb. Badger Hollow I and II estimated commercial operation dates are April 2021 and December 2022, respectively.