Income Taxes (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income tax provision components [Abstract]
Current payable: Federal									$ (891,000)	$ (1,508,000)	$ (6,053,000)
Current payable: State									(589,000)	8,213,000	436,000
Net-deferred: Federal									39,284,000	37,203,000	37,178,000
Net-deferred: State									10,600,000	1,163,000	7,618,000
Amortized investment tax credits									(219,000)	(212,000)	(260,000)
Total income tax provision	8,600,000	14,286,000	9,034,000	16,265,000	7,829,000	14,692,000	8,660,000	13,678,000	48,185,000	44,859,000	38,919,000
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%	5.00%
Amortized investment tax credits									(0.20%)	(0.20%)	(0.30%)
Credit for electricity from wind energy									(1.70%)	(1.50%)	(1.60%)
Domestic manufacturing deduction									0.00%	(0.20%)	0.30%
AFUDC Equity, net									(0.80%)	(0.70%)	(0.40%)
Other, net, individually significant									0.10%	0.00%	(0.30%)
Effective income tax rate									37.50%	37.50%	37.70%
Deferred tax liabilities (assets) on balance sheet [Abstract]
Property-related	312,903,000				263,881,000				312,903,000	263,881,000
Investment in ATC	36,140,000				32,696,000				36,140,000	32,696,000
Bond transactions	1,420,000				1,553,000				1,420,000	1,553,000
Pension and other postretirement benefits	57,847,000				34,478,000				57,847,000	34,478,000
Derivatives	22,331,000				26,361,000				22,331,000	26,361,000
Tax deductible prepayments	8,077,000				7,508,000				8,077,000	7,508,000
Other deferred tax liabilities	10,451,000				1,995,000				10,451,000	1,995,000
Gross deferred income tax liabilities	449,169,000				368,472,000				449,169,000	368,472,000
Future tax benefit	(4,092,000)				0				(4,092,000)	0
Accrued expenses	(32,091,000)				(17,195,000)				(32,091,000)	(17,195,000)
Pension and other postretirement benefits	(44,994,000)				(26,838,000)				(44,994,000)	(26,838,000)
Deferred tax regulatory account	(1,211,000)				(1,402,000)				(1,211,000)	(1,402,000)
Derivatives	(22,331,000)				(26,361,000)				(22,331,000)	(26,361,000)
Other deferred tax assets	(5,957,000)				(10,369,000)				(5,957,000)	(10,369,000)
Gross deferred income tax assets	(110,676,000)				(82,165,000)				(110,676,000)	(82,165,000)
Less valuation allowance	70,000				195,000				70,000	195,000
Net deferred income tax assets	(110,606,000)				(81,970,000)				(110,606,000)	(81,970,000)
Deferred income taxes	338,563,000				286,502,000				338,563,000	286,502,000
Operating loss deductions and tax credit carryforwards [Abstract]
State net tax operating loss deductions	1,400,000								1,400,000
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning balance				2,363,000				3,204,000	2,363,000	3,204,000	2,364,000
Additions based on tax positions related to current year									610,000	377,000	401,000
Additions based on tax positions related to prior year									618,000	424,000	580,000
Reductions based on tax positions related to the current year									0	(40,000)	0
Reductions based on tax positions related to prior years									(1,226,000)	(1,602,000)	(141,000)
Unrecognized tax benefits, ending balance	2,365,000				2,363,000				2,365,000	2,363,000	3,204,000
Interest on unrecognized tax benefits [Roll Forward]
Accrued interest on unrecognized tax benefits, beginning balance				101,000				314,000	101,000	314,000	216,000
Reduction in interest expense on uncertain tax positions									(97,000)	(275,000)	0
Interest expense on uncertain tax positions									88,000	62,000	98,000
Accrued interest on unrecognized tax benefits, ending balance	92,000				101,000				92,000	101,000	314,000
Unrecognized Tax Benefits
Unrecognized tax benefits	2,365,000				2,363,000				2,365,000	2,363,000	3,204,000
Unrecognized tax benefits permanent differences	0				0				0	0	0
Other deferred liabilities [Member]
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, ending balance	400,000				2,400,000				400,000	2,400,000
Unrecognized Tax Benefits
Unrecognized tax benefits	400,000				2,400,000				400,000	2,400,000
Deferred tax assets [Member]
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, ending balance	2,000,000								2,000,000
Unrecognized Tax Benefits
Unrecognized tax benefits	2,000,000								2,000,000
State [Member]
Operating loss deductions and tax credit carryforwards [Abstract]
Operating loss carryforward	16,500,000								16,500,000
Operating loss carryforward, deferred tax asset	800,000								800,000
Federal [Member]
Operating loss deductions and tax credit carryforwards [Abstract]
Tax credit carryforward, amount	5,400,000								5,400,000
Tax credit carryforward, deferred tax asset	5,400,000								5,400,000
MGE [Member]
Income tax provision components [Abstract]
Current payable: Federal									637,000	(448,000)	(5,030,000)
Current payable: State									(451,000)	8,322,000	613,000
Net-deferred: Federal									38,553,000	36,937,000	36,589,000
Net-deferred: State									10,625,000	1,223,000	7,523,000
Amortized investment tax credits									(219,000)	(212,000)	(260,000)
Total income tax provision									49,145,000	45,822,000	39,435,000
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%	5.00%
Amortized investment tax credits									(0.20%)	(0.20%)	(0.20%)
Credit for electricity from wind energy									(1.70%)	(1.50%)	(1.50%)
Domestic manufacturing deduction									0.00%	(0.20%)	0.30%
AFUDC Equity, net									(0.80%)	(0.70%)	(0.40%)
Other, net, individually significant									0.10%	0.00%	(0.50%)
Effective income tax rate									37.50%	37.50%	37.70%
Deferred tax liabilities (assets) on balance sheet [Abstract]
Property-related	312,807,000				263,881,000				312,807,000	263,881,000
Investment in ATC	29,156,000				27,073,000				29,156,000	27,073,000
Bond transactions	1,420,000				1,553,000				1,420,000	1,553,000
Pension and other postretirement benefits	57,847,000				34,478,000				57,847,000	34,478,000
Derivatives	22,331,000				26,361,000				22,331,000	26,361,000
Tax deductible prepayments	8,077,000				7,508,000				8,077,000	7,508,000
Other deferred tax liabilities	10,259,000				1,911,000				10,259,000	1,911,000
Gross deferred income tax liabilities	441,897,000				362,765,000				441,897,000	362,765,000
Future tax benefit	(4,092,000)				0				(4,092,000)	0
Accrued expenses	(32,091,000)				(17,195,000)				(32,091,000)	(17,195,000)
Pension and other postretirement benefits	(44,994,000)				(26,838,000)				(44,994,000)	(26,838,000)
Deferred tax regulatory account	(1,211,000)				(1,402,000)				(1,211,000)	(1,402,000)
Derivatives	(22,331,000)				(26,361,000)				(22,331,000)	(26,361,000)
Other deferred tax assets	(3,746,000)				(9,356,000)				(3,746,000)	(9,356,000)
Gross deferred income tax assets	(108,465,000)				(81,152,000)				(108,465,000)	(81,152,000)
Less valuation allowance	70,000				195,000				70,000	195,000
Net deferred income tax assets	(108,395,000)				(80,957,000)				(108,395,000)	(80,957,000)
Deferred income taxes	333,502,000				281,808,000				333,502,000	281,808,000
Operating loss deductions and tax credit carryforwards [Abstract]
State net tax operating loss deductions	1,400,000								1,400,000
MGE [Member] | State [Member]
Operating loss deductions and tax credit carryforwards [Abstract]
Operating loss carryforward	16,500,000								16,500,000
Operating loss carryforward, deferred tax asset	800,000								800,000
MGE [Member] | Federal [Member]
Operating loss deductions and tax credit carryforwards [Abstract]
Tax credit carryforward, amount	5,400,000								5,400,000
Tax credit carryforward, deferred tax asset	$ 5,400,000								$ 5,400,000