Income Taxes (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Income tax provision components [Abstract]
Current payable: Federal									$ (1,504,000)	$ 6,148,000	$ 8,008,000
Current payable: State									4,580,000	2,603,000	2,796,000
Net-deferred: Federal									30,115,000	20,811,000	14,408,000
Net-deferred: State									3,102,000	4,570,000	3,300,000
Amortized investment tax credits									(301,000)	(312,000)	(342,000)
Total income tax provision	5,274,000	12,495,000	7,572,000	10,651,000	6,030,000	12,990,000	6,472,000	8,328,000	35,992,000	33,820,000	28,170,000
Reconciliation of tax provision to statutory federal income tax rate [Abstract]
Statutory federal income tax rate									35.00%	35.00%	35.00%
State income taxes, net of federal benefit									5.10%	5.10%	5.10%
Amortized investment tax credits									(0.30%)	(0.30%)	(0.40%)
Credit for electricity from wind energy									(1.80%)	(1.70%)	(2.50%)
Domestic manufacturing deduction									(0.70%)	(0.40%)	(0.50%)
Other, net, individually insignificant									(0.20%)	(0.70%)	(1.10%)
Effective income tax rate									37.10%	37.00%	35.60%
Deferred tax liabilities (assets) on balance sheet [Abstract]
Property-related	188,235,000				159,348,000				188,235,000	159,348,000
Investment in ATC	26,896,000				24,467,000				26,896,000	24,467,000
Bond transactions	1,746,000				1,850,000				1,746,000	1,850,000
Pension and other postretirement benefits	68,352,000				38,417,000				68,352,000	38,417,000
Derivatives	16,498,000				7,771,000				16,498,000	7,771,000
Tax deductible prepayments	7,276,000				6,764,000				7,276,000	6,764,000
Other deferred tax liabilities	12,714,000				22,325,000				12,714,000	22,325,000
Gross deferred income tax liabilities	321,717,000				260,942,000				321,717,000	260,942,000
Accrued expenses	(28,458,000)				(27,529,000)				(28,458,000)	(27,529,000)
Pension and other postretirement benefits	(69,646,000)				(43,637,000)				(69,646,000)	(43,637,000)
Deferred tax regulatory account	(1,828,000)				(2,150,000)				(1,828,000)	(2,150,000)
Derivatives	(16,498,000)				(7,771,000)				(16,498,000)	(7,771,000)
Other deferred tax assets	(2,782,000)				(11,048,000)				(2,782,000)	(11,048,000)
Gross deferred income tax assets	(119,212,000)				(92,135,000)				(119,212,000)	(92,135,000)
Less valuation allowance	365,000				365,000				365,000	365,000
Net deferred income tax assets	(118,847,000)				(91,770,000)				(118,847,000)	(91,770,000)
Deferred income taxes	202,870,000				169,172,000				202,870,000	169,172,000
State net tax operating loss deductions [Abstract]
State net tax operating loss deductions	7,500,000								7,500,000
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning balance				4,377,000				176,000	4,377,000	176,000	176,000
Additions based on tax positions related to current year									128,000	386,000	0
Additions based on tax positions related to prior year									427,000	3,815,000	0
Reductions based on tax positions related to prior years									(2,568,000)	0	0
Unrecognized tax benefits, ending balance	2,364,000				4,377,000				2,364,000	4,377,000	176,000
Interest on unrecognized tax benefits [Roll Forward]
Accrued interest on unrecognized tax benefits, beginning balance				214,000				21,000	214,000	21,000	9,000
Interest expense on uncertain tax positions									2,000	193,000	12,000
Accrued interest on unrecognized tax benefits, ending balance	216,000				214,000				216,000	214,000	21,000
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefits permanent differences	200,000				200,000				200,000	200,000
Open tax years by major tax jurisdiction									MGE Energy and consolidated subsidiaries in federal return - 2007 through 2011, MGE Energy Wisconsin separate corporation return - 2007 through 2008, MGE Wisconsin separate corporation return - 2007 through 2008, MGE Energy Wisconsin combined reporting corporation return - 2009 through 2011
Regulatory Assets [Line Items]
Regulatory assets	213,182,000				122,817,000				213,182,000	122,817,000
Medicare Part D Subsidy [Member]
Regulatory Assets [Line Items]
Regulatory assets	2,900,000								2,900,000
MGE [Member]
Income tax provision components [Abstract]
Current payable: Federal									(607,000)	7,499,000	8,292,000
Current payable: State									4,658,000	2,764,000	2,855,000
Net-deferred: Federal									29,255,000	19,861,000	13,441,000
Net-deferred: State									3,034,000	4,493,000	3,065,000
Amortized investment tax credits									(301,000)	(312,000)	(342,000)
Total income tax provision									36,039,000	34,305,000	27,311,000
Reconciliation of tax provision to statutory federal income tax rate [Abstract]
Statutory federal income tax rate									35.00%	35.00%	35.00%
State income taxes, net of federal benefit									5.10%	5.10%	5.10%
Amortized investment tax credits									(0.30%)	(0.30%)	(0.40%)
Credit for electricity from wind energy									(1.80%)	(1.70%)	(2.60%)
Domestic manufacturing deduction									(0.70%)	(0.40%)	(0.50%)
Other, net, individually insignificant									(0.30%)	(0.70%)	(1.20%)
Effective income tax rate									37.00%	37.00%	35.40%
Deferred tax liabilities (assets) on balance sheet [Abstract]
Property-related	188,235,000				159,348,000				188,235,000	159,348,000
Investment in ATC	23,594,000				22,093,000				23,594,000	22,093,000
Bond transactions	1,746,000				1,850,000				1,746,000	1,850,000
Pension and other postretirement benefits	68,352,000				38,417,000				68,352,000	38,417,000
Derivatives	16,498,000				7,771,000				16,498,000	7,771,000
Tax deductible prepayments	7,276,000				6,764,000				7,276,000	6,764,000
Other deferred tax liabilities	12,697,000				22,308,000				12,697,000	22,308,000
Gross deferred income tax liabilities	318,398,000				258,551,000				318,398,000	258,551,000
Accrued expenses	(28,438,000)				(27,513,000)				(28,438,000)	(27,513,000)
Pension and other postretirement benefits	(69,646,000)				(43,637,000)				(69,646,000)	(43,637,000)
Deferred tax regulatory account	(1,828,000)				(2,150,000)				(1,828,000)	(2,150,000)
Derivatives	(16,498,000)				(7,771,000)				(16,498,000)	(7,771,000)
Other deferred tax assets	(2,783,000)				(11,048,000)				(2,783,000)	(11,048,000)
Gross deferred income tax assets	(119,193,000)				(92,119,000)				(119,193,000)	(92,119,000)
Less valuation allowance	365,000				365,000				365,000	365,000
Net deferred income tax assets	(118,828,000)				(91,754,000)				(118,828,000)	(91,754,000)
Deferred income taxes	199,570,000				166,797,000				199,570,000	166,797,000
State net tax operating loss deductions [Abstract]
State net tax operating loss deductions	$ 7,500,000								$ 7,500,000