Notes Payable to Banks, Commercial Paper, and Lines of Credit (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Short-term Debt [Line Items]
Short-term debt outstanding	$ 0	$ 22,500,000	$ 64,500,000
Weighted-average interest rate	0.00%	1.36%	0.53%
Maximum short-term borrowings	28,500,000	122,500,000	134,000,000
Average short-term borrowings	3,410,000	58,080,000	105,015,000
Weighted-average interest rate during the year	1.43%	0.56%	0.63%
Long-term debt instruments [Line Items]
Other Long-Term Debt	195,973,000	168,473,000
5.04%, due 2040 [Member]
Long-term debt instruments [Line Items]
Other Long-Term Debt	46,806,000	48,473,000	50,000,000
Interest rate on secured long-term debt	5.04%	5.04%
Line of Credit [Member]
Credit facilities [Line Items]
Lines of credit borrowing capacity	115,000,000	115,000,000	195,000,000
MGE [Member]
Short-term Debt [Line Items]
Commercial paper outstanding	0	3,500,000	33,500,000
Short-term debt outstanding	0	3,500,000
Weighted-average interest rate	0.00%	0.25%	0.20%
Maximum short-term borrowings	9,500,000	41,000,000	60,500,000
Average short-term borrowings	349,000	20,720,000	26,900,000
Weighted-average interest rate during the year	0.25%	0.24%	0.31%
Long-term debt instruments [Line Items]
Other Long-Term Debt	195,973,000	168,473,000
MGE [Member] | 5.04%, due 2040 [Member]
Long-term debt instruments [Line Items]
Other Long-Term Debt	46,806,000	48,473,000
Interest rate on secured long-term debt	5.04%	5.04%
MGE [Member] | Line of Credit [Member]
Credit facilities [Line Items]
Lines of credit borrowing capacity	75,000,000	75,000,000	75,000,000
Line of credit borrowings	0
Allowable ratio of indebtedness to capitalization	65.00%
Percent of equity interest parent must hold	100.00%
Disallowable ownership of stock by outside party	30.00%
MGE Energy [Member]
Short-term Debt [Line Items]
Short-term debt outstanding	0	19,000,000
MGE Energy [Member] | Line of Credit [Member]
Credit facilities [Line Items]
Lines of credit borrowing capacity	40,000,000
Line of credit borrowings	$ 0
Allowable ratio of indebtedness to capitalization	65.00%
Percent of equity interest parent must hold	100.00%
Disallowable ownership of stock by outside party	30.00%