Variable Interest Entities (Details) (USD $)	Dec. 31, 2011	Dec. 31, 2010
Balance sheet accounts [Line Items]
Property, plant, and equipment, net	$ 961,511,000	$ 857,572,000
Construction work in progress	34,055,000	110,435,000
Accrued interest and taxes	4,085,000	3,990,000
Deferred income taxes	321,717,000	260,942,000
Long-term debt	364,473,000
MGE Power Elm Road [Member]
Balance sheet accounts [Line Items]
Property, plant, and equipment, net	189,163,000	120,122,000
Construction work in progress	732,000	72,193,000
Accrued interest and taxes	4,348,000	1,761,000
Deferred income taxes	15,553,000	11,067,000
Long-term debt	75,972,000	48,473,000
Noncontrolling interest	70,966,000	99,390,000
Long-term debt instruments [Line Items]
Debt service coverage ratio range, high	1.25
Debt service coverage ratio range, low	1.00
Carrying costs recovered [Line Items]
Total carrying costs	62,200,000
Total capitalized interest	17,000,000
Total capitalized cost of equity	45,200,000
MGE Power West Campus [Member]
Balance sheet accounts [Line Items]
Property, plant, and equipment, net	91,355,000	93,631,000
Affiliate receivables	7,416,000	7,945,000
Accrued interest and taxes	4,689,000	2,409,000
Deferred income taxes	20,659,000	19,985,000
Long-term debt	50,000,000	50,000,000
Noncontrolling interest	30,676,000	32,675,000
Long-term debt instruments [Line Items]
Debt service coverage ratio range, high	1.25
Debt service coverage ratio range, low	1.00
Debt to total capitalization ratio range, low	0.65
Debt to total capitalization ratio range, high	1.00
Carrying costs recovered [Line Items]
Carrying costs recovered	$ 12,100,000
Elm Road Units [Member] | MGE Power Elm Road [Member]
Elm Road Units ownership [Line Items]
Ownership interest in jointly owned plant	8.33%
Plant capacity - MW	615