Long-Term Debt (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Long-term Debt [Line Items]
First Mortgage Bonds	$ 1,200,000	$ 1,200,000
Tax Exempt Debt	19,300,000	47,300,000
Medium-term Notes	120,000,000	120,000,000
Other Long-Term Debt	221,305,000	195,973,000
Long-term debt due within one year	(3,013,000)	(2,667,000)
Unamortized discount	(301,000)	(903,000)
Total Long-Term Debt	358,491,000	360,903,000
Long-Term Debt Maturities [Abstract]
Long term debt maturities in next twelve months	3,013,000
Long term debt maturities in year two	4,102,000
Long term debt maturities in year three	4,182,000
Long term debt maturities in year four	4,267,000
Long term debt maturities in year five	54,357,000
Long term debt maturities after year five	291,884,000
Total	361,805,000
Remarketing of debt [Abstract]
Interest rate before remarketing	4.875%
Interest rate after remarketing	3.45%
7.70%, 2028 Series [Member]
Long-term Debt [Line Items]
First Mortgage Bonds	1,200,000	1,200,000
Interest rate on secured long-term debt	7.70%	7.70%
Available for dividends under first mortgage bond covenants	281,600,000
3.45% and 4.875%, respectively, 2027 Series [Member]
Long-term Debt [Line Items]
Tax Exempt Debt	19,300,000	19,300,000
Interest rate on secured long-term debt	3.45%	4.875%
5.875% 2034 Series [Member]
Long-term Debt [Line Items]
Tax Exempt Debt	0	28,000,000
Interest rate on secured long-term debt	0.00%	5.875%
5.26%, due 2017 [Member]
Long-term Debt [Line Items]
Medium-term Notes	20,000,000	20,000,000
Interest rate on secured long-term debt	5.26%	5.26%
5.25%, due 2017 [Member]
Long-term Debt [Line Items]
Medium-term Notes	30,000,000	30,000,000
Interest rate on secured long-term debt	5.25%	5.25%
7.12%, due 2032 [Member]
Long-term Debt [Line Items]
Medium-term Notes	25,000,000	25,000,000
Interest rate on secured long-term debt	7.12%	7.12%
6.12%, due 2028 [Member]
Long-term Debt [Line Items]
Medium-term Notes	20,000,000	20,000,000
Interest rate on secured long-term debt	6.12%	6.12%
6.247%, due 2037 [Member]
Long-term Debt [Line Items]
Medium-term Notes	25,000,000	25,000,000
Interest rate on secured long-term debt	6.247%	6.247%
5.59%, due 2018 [Member]
Long-term Debt [Line Items]
Other Long-Term Debt	40,000,000	40,000,000
Interest rate on secured long-term debt	5.59%	5.59%
Disallowable ownership of stock by outside party	30.00%
Allowable ratio of indebtedness to capitalization	65.00%
Allowable ratio of priority debt to assets	20.00%
5.68%, due 2033 [Member]
Long-term Debt [Line Items]
Other Long-Term Debt	30,000,000	30,000,000
Interest rate on secured long-term debt	5.68%	5.68%
5.19%, due 2033 [Member]
Long-term Debt [Line Items]
Other Long-Term Debt	20,000,000	20,000,000
Interest rate on secured long-term debt	5.19%	5.19%
5.04%, due 2040 [Member]
Long-term Debt [Line Items]
Other Long-Term Debt	45,138,000	46,806,000
Interest rate on secured long-term debt	5.04%	5.04%
3.38%, due 2020 [Member]
Long-term Debt [Line Items]
Other Long-Term Debt	15,000,000	15,000,000
Interest rate on secured long-term debt	3.38%	3.38%
Disallowable ownership of stock by outside party	30.00%
Allowable ratio of indebtedness to capitalization	65.00%
Allowable ratio of priority debt to assets	20.00%
5.26%, due 2040 [Member]
Long-term Debt [Line Items]
Other Long-Term Debt	15,000,000	15,000,000
Interest rate on secured long-term debt	5.26%	5.26%
Disallowable ownership of stock by outside party	30.00%
Allowable ratio of indebtedness to capitalization	65.00%
Allowable ratio of priority debt to assets	20.00%
4.74%, due 2041 [Member]
Long-term Debt [Line Items]
Other Long-Term Debt	28,167,000	29,167,000
Interest rate on secured long-term debt	4.74%	4.74%
4.38%, due 2042 [Member]
Long-term Debt [Line Items]
Other Long-Term Debt	28,000,000	0
Interest rate on secured long-term debt	4.38%	0.00%
MGE [Member]
Long-term Debt [Line Items]
First Mortgage Bonds	1,200,000	1,200,000
Tax Exempt Debt	19,300,000	47,300,000
Medium-term Notes	120,000,000	120,000,000
Other Long-Term Debt	221,305,000	195,973,000
Long-term debt due within one year	(3,013,000)	(2,667,000)
Unamortized discount	(301,000)	(903,000)
Total Long-Term Debt	358,491,000	360,903,000
Available for dividends under first mortgage bond covenants	281,600,000
Long-Term Debt Maturities [Abstract]
Long term debt maturities in next twelve months	3,013,000
Long term debt maturities in year two	4,102,000
Long term debt maturities in year three	4,182,000
Long term debt maturities in year four	4,267,000
Long term debt maturities in year five	54,357,000
Long term debt maturities after year five	291,884,000
Total	361,805,000
Remarketing of debt [Abstract]
Interest rate before remarketing	4.875%
Interest rate after remarketing	3.45%
MGE [Member] | 7.70%, 2028 Series [Member]
Long-term Debt [Line Items]
First Mortgage Bonds	1,200,000	1,200,000
Interest rate on secured long-term debt	7.70%	7.70%
Available for dividends under first mortgage bond covenants	281,600,000
MGE [Member] | 3.45% and 4.875%, respectively, 2027 Series [Member]
Long-term Debt [Line Items]
Tax Exempt Debt	19,300,000	19,300,000
Interest rate on secured long-term debt	3.45%	4.875%
MGE [Member] | 5.875% 2034 Series [Member]
Long-term Debt [Line Items]
Tax Exempt Debt	0	28,000,000
Interest rate on secured long-term debt	0.00%	5.875%
MGE [Member] | 5.26%, due 2017 [Member]
Long-term Debt [Line Items]
Medium-term Notes	20,000,000	20,000,000
Interest rate on secured long-term debt	5.26%	5.26%
MGE [Member] | 5.25%, due 2017 [Member]
Long-term Debt [Line Items]
Medium-term Notes	30,000,000	30,000,000
Interest rate on secured long-term debt	5.25%	5.25%
MGE [Member] | 7.12%, due 2032 [Member]
Long-term Debt [Line Items]
Medium-term Notes	25,000,000	25,000,000
Interest rate on secured long-term debt	7.12%	7.12%
MGE [Member] | 6.12%, due 2028 [Member]
Long-term Debt [Line Items]
Medium-term Notes	20,000,000	20,000,000
Interest rate on secured long-term debt	6.12%	6.12%
MGE [Member] | 6.247%, due 2037 [Member]
Long-term Debt [Line Items]
Medium-term Notes	25,000,000	25,000,000
Interest rate on secured long-term debt	6.247%	6.247%
MGE [Member] | 5.59%, due 2018 [Member]
Long-term Debt [Line Items]
Other Long-Term Debt	40,000,000	40,000,000
Interest rate on secured long-term debt	5.59%	5.59%
Disallowable ownership of stock by outside party	30.00%
Allowable ratio of indebtedness to capitalization	65.00%
Allowable ratio of priority debt to assets	20.00%
MGE [Member] | 5.68%, due 2033 [Member]
Long-term Debt [Line Items]
Other Long-Term Debt	30,000,000	30,000,000
Interest rate on secured long-term debt	5.68%	5.68%
MGE [Member] | 5.19%, due 2033 [Member]
Long-term Debt [Line Items]
Other Long-Term Debt	20,000,000	20,000,000
Interest rate on secured long-term debt	5.19%	5.19%
MGE [Member] | 5.04%, due 2040 [Member]
Long-term Debt [Line Items]
Other Long-Term Debt	45,138,000	46,806,000
Interest rate on secured long-term debt	5.04%	5.04%
MGE [Member] | 3.38%, due 2020 [Member]
Long-term Debt [Line Items]
Other Long-Term Debt	15,000,000	15,000,000
Interest rate on secured long-term debt	3.38%	3.38%
Disallowable ownership of stock by outside party	30.00%
Allowable ratio of indebtedness to capitalization	65.00%
Allowable ratio of priority debt to assets	20.00%
MGE [Member] | 5.26%, due 2040 [Member]
Long-term Debt [Line Items]
Other Long-Term Debt	15,000,000	15,000,000
Interest rate on secured long-term debt	5.26%	5.26%
Disallowable ownership of stock by outside party	30.00%
Allowable ratio of indebtedness to capitalization	65.00%
Allowable ratio of priority debt to assets	20.00%
MGE [Member] | 4.74%, due 2041 [Member]
Long-term Debt [Line Items]
Other Long-Term Debt	28,167,000	29,167,000
Interest rate on secured long-term debt	4.74%	4.74%
MGE [Member] | 4.38%, due 2042 [Member]
Long-term Debt [Line Items]
Other Long-Term Debt	28,000,000	0
Interest rate on secured long-term debt	4.38%	0.00%
MGE Power Elm Road [Member]
Long-term Debt [Line Items]
Debt service coverage ratio range, high	1.25
Debt service coverage ratio range, low	1.00
Long-Term Debt Maturities [Abstract]
Total	73,306,000	75,972,000
MGE Power Elm Road [Member] | 5.04%, due 2040 [Member]
Long-term Debt [Line Items]
Debt service coverage ratio range, high	1.25
Debt service coverage ratio range, low	1.00
MGE Power Elm Road [Member] | 4.74%, due 2041 [Member]
Long-term Debt [Line Items]
Debt service coverage ratio range, high	1.25
Debt service coverage ratio range, low	1.00
MGE Power West Campus [Member]
Long-term Debt [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
Long-Term Debt Maturities [Abstract]
Total	$ 50,000,000	$ 50,000,000
MGE Power West Campus [Member] | 5.68%, due 2033 [Member]
Long-term Debt [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
MGE Power West Campus [Member] | 5.19%, due 2033 [Member]
Long-term Debt [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