Income Taxes (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income tax provision components [Abstract]
Current payable: Federal									$ (1,508,000)	$ (6,053,000)	$ (1,504,000)
Current payable: State									8,213,000	436,000	4,580,000
Net-deferred: Federal									37,203,000	37,178,000	30,115,000
Net-deferred: State									1,163,000	7,618,000	3,102,000
Amortized investment tax credits									(212,000)	(260,000)	(301,000)
Total income tax provision	7,829,000	14,692,000	8,660,000	13,678,000	6,208,000	14,253,000	8,596,000	9,862,000	44,859,000	38,919,000	35,992,000
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%	5.00%	5.10%
Amortized investment tax credits									(0.20%)	(0.30%)	(0.30%)
Credit for electricity from wind energy									(1.50%)	(1.60%)	(1.80%)
Domestic manufacturing deduction									(0.20%)	0.30%	(0.70%)
AFUDC Equity, net									(0.70%)	(0.40%)	0.00%
Other, net, individually significant									0.00%	(0.30%)	(0.20%)
Effective income tax rate									37.50%	37.70%	37.10%
Deferred tax liabilities (assets) on balance sheet [Abstract]
Property-related	263,881,000				248,545,000				263,881,000	248,545,000
Investment in ATC	32,696,000				29,147,000				32,696,000	29,147,000
Bond transactions	1,553,000				1,643,000				1,553,000	1,643,000
Pension and other postretirement benefits	34,478,000				82,072,000				34,478,000	82,072,000
Derivatives	26,361,000				29,134,000				26,361,000	29,134,000
Tax deductible prepayments	7,508,000				7,233,000				7,508,000	7,233,000
Other deferred tax liabilities	1,995,000				10,662,000				1,995,000	10,662,000
Gross deferred income tax liabilities	368,472,000				408,436,000				368,472,000	408,436,000
Future federal tax benefit	0				(25,626,000)				0	(25,626,000)
Accrued expenses	(17,195,000)				(30,277,000)				(17,195,000)	(30,277,000)
Pension and other postretirement benefits	(26,838,000)				(69,941,000)				(26,838,000)	(69,941,000)
Deferred tax regulatory account	(1,402,000)				(1,551,000)				(1,402,000)	(1,551,000)
Derivatives	(26,361,000)				(29,134,000)				(26,361,000)	(29,134,000)
Other deferred tax assets	(10,369,000)				(5,345,000)				(10,369,000)	(5,345,000)
Gross deferred income tax assets	(82,165,000)				(161,874,000)				(82,165,000)	(161,874,000)
Less valuation allowance	195,000				365,000				195,000	365,000
Net deferred income tax assets	(81,970,000)				(161,509,000)				(81,970,000)	(161,509,000)
Deferred income taxes	286,502,000				246,927,000				286,502,000	246,927,000
Operating loss deductions and tax credit carryforwards [Abstract]
Operating loss carryforward	0				63,400,000				0	63,400,000
Operating loss carryforward, deferred tax asset					22,200,000					22,200,000
Tax credit carryforward, amount	0				3,400,000				0	3,400,000
Tax credit carryforward, deferred tax asset					3,400,000					3,400,000
State net tax operating loss deductions	4,000,000								4,000,000
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning balance				3,204,000				2,364,000	3,204,000	2,364,000	4,377,000
Additions based on tax positions related to current year									377,000	401,000	128,000
Additions based on tax positions related to prior year									424,000	580,000	427,000
Reductions based on tax positions related to the current year									(40,000)	0	0
Reductions based on tax positions related to prior years									(1,602,000)	(141,000)	(2,568,000)
Unrecognized tax benefits, ending balance	2,363,000				3,204,000				2,363,000	3,204,000	2,364,000
Interest on unrecognized tax benefits [Roll Forward]
Accrued interest on unrecognized tax benefits, beginning balance				314,000				216,000	314,000	216,000	214,000
Reduction in interest expense on uncertain tax positions									(275,000)	0	0
Interest expense on uncertain tax positions									62,000	98,000	2,000
Accrued interest on unrecognized tax benefits, ending balance	101,000				314,000				101,000	314,000	216,000
Unrecognized Tax Benefits
Unrecognized tax benefits permanent differences	0				0				0	0	200,000
MGE [Member]
Income tax provision components [Abstract]
Current payable: Federal									(448,000)	(5,030,000)	(607,000)
Current payable: State									8,322,000	613,000	4,658,000
Net-deferred: Federal									36,937,000	36,589,000	29,255,000
Net-deferred: State									1,223,000	7,523,000	3,034,000
Amortized investment tax credits									(212,000)	(260,000)	(301,000)
Total income tax provision									45,822,000	39,435,000	36,039,000
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%	5.00%	5.10%
Amortized investment tax credits									(0.20%)	(0.20%)	(0.30%)
Credit for electricity from wind energy									(1.50%)	(1.50%)	(1.80%)
Domestic manufacturing deduction									(0.20%)	0.30%	(0.70%)
AFUDC Equity, net									(0.70%)	(0.40%)	0.00%
Other, net, individually significant									0.00%	(0.50%)	(0.30%)
Effective income tax rate									37.50%	37.70%	37.00%
Deferred tax liabilities (assets) on balance sheet [Abstract]
Property-related	263,881,000				248,545,000				263,881,000	248,545,000
Investment in ATC	27,073,000				24,993,000				27,073,000	24,993,000
Bond transactions	1,553,000				1,643,000				1,553,000	1,643,000
Pension and other postretirement benefits	34,478,000				82,072,000				34,478,000	82,072,000
Derivatives	26,361,000				29,134,000				26,361,000	29,134,000
Tax deductible prepayments	7,508,000				7,233,000				7,508,000	7,233,000
Other deferred tax liabilities	1,911,000				10,634,000				1,911,000	10,634,000
Gross deferred income tax liabilities	362,765,000				404,254,000				362,765,000	404,254,000
Future federal tax benefit	0				(25,899,000)				0	(25,899,000)
Accrued expenses	(17,195,000)				(30,228,000)				(17,195,000)	(30,228,000)
Pension and other postretirement benefits	(26,838,000)				(69,941,000)				(26,838,000)	(69,941,000)
Deferred tax regulatory account	(1,402,000)				(1,551,000)				(1,402,000)	(1,551,000)
Derivatives	(26,361,000)				(29,134,000)				(26,361,000)	(29,134,000)
Other deferred tax assets	(9,356,000)				(4,940,000)				(9,356,000)	(4,940,000)
Gross deferred income tax assets	(81,152,000)				(161,693,000)				(81,152,000)	(161,693,000)
Less valuation allowance	195,000				365,000				195,000	365,000
Net deferred income tax assets	(80,957,000)				(161,328,000)				(80,957,000)	(161,328,000)
Deferred income taxes	281,808,000				242,926,000				281,808,000	242,926,000
Operating loss deductions and tax credit carryforwards [Abstract]
Operating loss carryforward	0				64,200,000				0	64,200,000
Operating loss carryforward, deferred tax asset					22,500,000					22,500,000
Tax credit carryforward, amount	0				3,400,000				0	3,400,000
Tax credit carryforward, deferred tax asset					3,400,000					3,400,000
State net tax operating loss deductions	$ 4,000,000								$ 4,000,000