Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income tax provision components [Abstract]
Current payable: Federal									$ 16,837	$ (891)	$ (1,508)
Current payable: State									2,774	(589)	8,213
Net-deferred: Federal									15,951	39,284	37,203
Net-deferred: State									5,976	10,600	1,163
Amortized investment tax credits									(175)	(219)	(212)
Total income tax provision	$ 7,417	$ 15,351	$ 8,008	$ 10,587	$ 8,600	$ 14,286	$ 9,034	$ 16,265	$ 41,363	$ 48,185	$ 44,859
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									5.20%	5.10%	5.10%
Amortized investment tax credits									(0.20%)	(0.20%)	(0.20%)
Credit for electricity from wind energy									(1.80%)	(1.70%)	(1.50%)
Domestic manufacturing deduction									(1.40%)	0.00%	(0.20%)
AFUDC Equity, net									(0.10%)	(0.80%)	(0.70%)
Other, net, individually significant									0.00%	0.10%	0.00%
Effective income tax rate									36.70%	37.50%	37.50%
Deferred tax liabilities (assets) on balance sheet [Abstract]
Property-related												$ 327,918	$ 312,903
Investment in ATC												38,213	36,140
Bond transactions												1,422	1,420
Pension and other postretirement benefits												57,697	57,847
Derivatives												21,660	22,331
Tax deductible prepayments												8,011	8,077
Other deferred tax liabilities												14,997	10,451
Gross deferred income tax liabilities												469,918	449,169
Future tax benefit												0	(4,092)
Accrued expenses												(21,391)	(32,091)
Pension and other postretirement benefits												(46,582)	(44,994)
Deferred tax regulatory account												(1,047)	(1,211)
Derivatives												(21,660)	(22,331)
Other deferred tax assets												(18,523)	(5,957)
Gross deferred income tax assets												(109,203)	(110,676)
Less valuation allowance												70	70
Net deferred income tax assets												(109,133)	(110,606)
Deferred income taxes												360,785	338,563
Operating loss deductions and tax credit carryforwards [Abstract]
State net tax operating loss deductions												1,400
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning balance				2,365				2,363	$ 2,365	$ 2,363	$ 3,204
Additions based on tax positions related to current year									488	610	377
Additions based on tax positions related to prior year									520	618	424
Reductions based on tax positions related to the current year									0	0	(40)
Reductions based on tax positions related to prior years									(845)	(1,226)	(1,602)
Unrecognized tax benefits, ending balance	2,528				2,365				2,528	2,365	2,363
Interest on unrecognized tax benefits [Roll Forward]
Accrued interest on unrecognized tax benefits, beginning balance				92				101	92	101	314
Reduction in interest expense on uncertain tax positions									(102)	(97)	(275)
Interest expense on uncertain tax positions									321	88	62
Accrued interest on unrecognized tax benefits, ending balance	311				92				311	92	101
Unrecognized Tax Benefits
Unrecognized tax benefits	2,528			2,365	2,365			$ 2,363	2,365	2,365	2,363	2,528	2,365	$ 2,363
Unrecognized tax benefits permanent differences												0	0	$ 0
Other deferred liabilities [Member]
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning balance				400					400
Unrecognized tax benefits, ending balance	2,500				400				2,500	400
Unrecognized Tax Benefits
Unrecognized tax benefits	$ 2,500			400	400				2,500	400		2,500	400
Deferred tax assets [Member]
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning balance				2,000					2,000
Unrecognized tax benefits, ending balance					2,000					2,000
Unrecognized Tax Benefits
Unrecognized tax benefits				2,000	2,000				2,000	2,000			2,000
Current assets [Member] | Accounting Standards Update 2015-17 - Deferred Income Taxes [Member]
New Accounting Pronouncement Or Change In Accounting Principle Retrospective Adjustments [Abstract]
New accounting pronouncement, effect of adoption, change in assets (deferred income taxes)										(3,500)
Deferred liabilities [Member] | Accounting Standards Update 2015-17 - Deferred Income Taxes [Member]
New Accounting Pronouncement Or Change In Accounting Principle Retrospective Adjustments [Abstract]
New accounting pronouncement, effect of adoption, change in liabilities (deferred income taxes)										(3,500)
State [Member]
Operating loss deductions and tax credit carryforwards [Abstract]
Operating loss carryforward												0	16,500
Operating loss carryforward, deferred tax asset												0	800
Federal [Member]
Operating loss deductions and tax credit carryforwards [Abstract]
Tax credit carryforward, amount												0	5,400
Tax credit carryforward, deferred tax asset												0	5,400
MGE [Member]
Income tax provision components [Abstract]
Current payable: Federal									19,295	637	(448)
Current payable: State									3,443	(451)	8,322
Net-deferred: Federal									13,538	38,553	36,937
Net-deferred: State									5,305	10,625	1,223
Amortized investment tax credits									(175)	(219)	(212)
Total income tax provision									$ 41,406	$ 49,145	$ 45,822
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									5.20%	5.10%	5.10%
Amortized investment tax credits									(0.20%)	(0.20%)	(0.20%)
Credit for electricity from wind energy									(1.80%)	(1.70%)	(1.50%)
Domestic manufacturing deduction									(1.40%)	0.00%	(0.20%)
AFUDC Equity, net									(0.10%)	(0.80%)	(0.70%)
Other, net, individually significant									0.00%	0.10%	0.00%
Effective income tax rate									36.70%	37.50%	37.50%
Deferred tax liabilities (assets) on balance sheet [Abstract]
Property-related												327,822	312,807
Investment in ATC												30,382	29,156
Bond transactions												1,422	1,420
Pension and other postretirement benefits												57,697	57,847
Derivatives												21,660	22,331
Tax deductible prepayments												8,011	8,077
Other deferred tax liabilities												14,831	10,259
Gross deferred income tax liabilities												461,825	441,897
Future tax benefit												0	(4,092)
Accrued expenses												(21,391)	(32,091)
Pension and other postretirement benefits												(46,582)	(44,994)
Deferred tax regulatory account												(1,047)	(1,211)
Derivatives												(21,660)	(22,331)
Other deferred tax assets												(18,589)	(3,746)
Gross deferred income tax assets												(109,269)	(108,465)
Less valuation allowance												70	70
Net deferred income tax assets												(109,199)	(108,395)
Deferred income taxes												352,626	333,502
Operating loss deductions and tax credit carryforwards [Abstract]
State net tax operating loss deductions												1,400
MGE [Member] | Deferred tax assets [Member]
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning balance				2,000					$ 2,000
Unrecognized tax benefits, ending balance					2,000					$ 2,000
Unrecognized Tax Benefits
Unrecognized tax benefits				$ 2,000	$ 2,000				$ 2,000	2,000			2,000
MGE [Member] | Current assets [Member] | Accounting Standards Update 2015-17 - Deferred Income Taxes [Member]
New Accounting Pronouncement Or Change In Accounting Principle Retrospective Adjustments [Abstract]
New accounting pronouncement, effect of adoption, change in assets (deferred income taxes)										(1,300)
MGE [Member] | Deferred liabilities [Member] | Accounting Standards Update 2015-17 - Deferred Income Taxes [Member]
New Accounting Pronouncement Or Change In Accounting Principle Retrospective Adjustments [Abstract]
New accounting pronouncement, effect of adoption, change in liabilities (deferred income taxes)										$ (1,300)
MGE [Member] | State [Member]
Operating loss deductions and tax credit carryforwards [Abstract]
Operating loss carryforward												0	16,500
Operating loss carryforward, deferred tax asset												0	800
MGE [Member] | Federal [Member]
Operating loss deductions and tax credit carryforwards [Abstract]
Tax credit carryforward, amount												0	5,400
Tax credit carryforward, deferred tax asset												$ 0	$ 5,400