Joint Plant Ownership (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) operatingunit MW	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jan. 01, 2018
Jointly Owned Utility Plant Interests [Line Items]
Assets held for sale	$ 8,817	$ 14,813
MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Assets held for sale	8,817	14,813
Columbia Units [Member]
Jointly Owned Plants, Net Plant Ownership [Abstract]
Plant	274,450	270,898
Accumulated depreciation	(87,144)	(81,935)
Property, plant, and equipment, net	187,306	188,963
Construction work in progress	20,382	21,120
Total property, plant, and equipment	$ 207,688	$ 210,083
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	221
Jointly owned utility plant, ownership interest	20.40%	22.00%
Assets held for sale	$ 8,800	$ 14,800
Jointly owned utility plant, fuel, operating, and maintenance expense	36,200	39,500	$ 38,200
Columbia Units [Member] | MGE [Member] | Subsequent Event [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, ownership interest				19.40%
Elm Road Units [Member] | MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, fuel, operating, and maintenance expense	$ 17,300	21,200	20,900
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	$ 206,063	204,292
Accumulated depreciation	(30,061)	(28,790)
Property, plant, and equipment, net	176,002	175,502
Construction work in progress	1,225	3,241
Total property, plant, and equipment	$ 177,227	178,743
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%
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	$ 5,300	5,500	$ 3,700
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	$ 111,386	111,330
Accumulated depreciation	(31,104)	(29,742)
Property, plant, and equipment, net	80,282	81,588
Construction work in progress	610	1,009
Total property, plant, and equipment	$ 80,892	$ 82,597