Joint Plant Ownership (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) MW operatingunit Turbines MW	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2019
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, ownership interest	12.80%
Assets held for sale	$ 3,080,000	$ 8,817,000
Forward Wind Turbines | Turbines	86
Forward Wind Proportionate Purchase Price	$ 23,000,000
Forward Wind Aggregate Purchase Price	174,000,000
MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Assets held for sale	3,080,000	8,817,000
Columbia Units [Member]
Jointly Owned Plants, Net Plant Ownership [Abstract]
Plant	292,157,000	274,450,000
Accumulated depreciation	(94,766,000)	(87,144,000)
Property, plant, and equipment, net	197,391,000	187,306,000
Construction work in progress	2,021,000	20,382,000
Total property, plant, and equipment	$ 199,412,000	$ 207,688,000
Columbia Units [Member] | MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, proportion of total net summer generating capability	29.00%
Jointly owned utility plant, plant capacity (in MW) | MW	219
Jointly owned utility plant, ownership interest	19.40%	20.40%
Assets held for sale	$ 3,100,000	$ 8,800,000
Jointly owned utility plant, fuel, operating, and maintenance expense	36,500,000	36,200,000	$ 39,500,000
Columbia Units [Member] | MGE [Member] | Subsequent Event [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, ownership interest				19.10%
Elm Road Units [Member] | MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, fuel, operating, and maintenance expense	$ 17,600,000	17,300,000	21,200,000
Elm Road Units [Member] | MGE Power Elm Road [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, proportion of total net summer generating capability	14.00%
Jointly owned utility plant, proportion of total net summer generating capability, plant capacity (in MW) | MW	106
Jointly owned utility plant, number of operating units | operatingunit	2
Jointly Owned Plants, Net Plant Ownership [Abstract]
Plant	$ 207,361,000	206,063,000
Accumulated depreciation	(33,897,000)	(30,061,000)
Property, plant, and equipment, net	173,464,000	176,002,000
Construction work in progress	530,000	1,225,000
Total property, plant, and equipment	$ 173,994,000	177,227,000
Elm Road Unit 1 [Member] | MGE Power Elm Road [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, plant capacity (in MW) | MW	615
Jointly owned utility plant, ownership interest	8.33%
Elm Road Unit 2 [Member] | MGE Power Elm Road [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, plant capacity (in MW) | MW	615
Jointly owned utility plant, ownership interest	8.33%
Forward Wind [Member]
Jointly Owned Plants, Net Plant Ownership [Abstract]
Plant	$ 33,929,000
Accumulated depreciation	(12,530,000)
Property, plant, and equipment, net	21,399,000
Construction work in progress	67,000
Total property, plant, and equipment	$ 21,466,000
Forward Wind [Member] | MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, plant capacity (in MW) | MW	18
Jointly owned utility plant, ownership interest	12.80%
Jointly owned utility plant, fuel, operating, and maintenance expense	$ 500,000
Forward Wind Turbines | MW	86
Forward Wind Proportionate Purchase Price	$ 23,000,000
Forward Wind Purchase Power Energy Output	12.80%
Forward Wind [Member] | Joint Owners [Member]
Jointly Owned Utility Plant Interests [Line Items]
Forward Wind Aggregate Purchase Price	$ 174,000,000
West Campus [Member] | UW [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, ownership interest	45.00%
Jointly owned utility plant, fuel, operating, and maintenance expense	$ 6,300,000	5,300,000	$ 5,500,000
West Campus [Member] | MGE Power West Campus [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, ownership interest	55.00%
Jointly Owned Plants, Net Plant Ownership [Abstract]
Plant	$ 113,328,000	111,386,000
Accumulated depreciation	(32,055,000)	(31,104,000)
Property, plant, and equipment, net	81,273,000	80,282,000
Construction work in progress	478,000	610,000
Total property, plant, and equipment	$ 81,751,000	$ 80,892,000