Income Taxes (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income tax provision components [Abstract]
Current payable: Federal										$ 16,908	$ 16,837	$ (891)
Current payable: State										3,287	2,774	(589)
Net-deferred: Federal										17,571	15,951	39,284
Net-deferred: State										4,850	5,976	10,600
Amortized investment tax credits										(103)	(175)	(219)
Total income tax provision		$ 7,570	$ 15,714	$ 9,284	$ 9,945	$ 7,417	$ 15,351	$ 8,008	$ 10,587	$ 42,513	$ 41,363	$ 48,185
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%	5.20%	5.10%
Amortized investment tax credits										(0.10%)	(0.20%)	(0.20%)
Credit for electricity from wind energy										(1.60%)	(1.80%)	(1.70%)
Domestic manufacturing deduction										(1.30%)	(1.40%)	0.00%
AFUDC equity, net										(0.20%)	(0.10%)	(0.80%)
Other, net, individually significant										(0.90%)	0.00%	0.10%
Effective income tax rate										36.00%	36.70%	37.50%
Deferred tax liabilities (assets) on balance sheet [Abstract]
Property-related													$ 344,721	$ 327,918
Investment in ATC	[1]												71,898	38,213
Bond transactions													1,324	1,422
Pension and other postretirement benefits													63,516	57,697
Derivatives													20,304	21,660
Tax deductible prepayments													8,404	8,011
Other deferred tax liabilities													16,606	14,997
Gross deferred income tax liabilities													526,773	469,918
Investment in ATC	[1]												(31,212)	0
Accrued expenses													(22,410)	(21,391)
Pension and other postretirement benefits													(48,860)	(46,582)
Deferred tax regulatory account													(903)	(1,047)
Derivatives													(20,304)	(21,660)
Other deferred tax assets													(19,340)	(18,523)
Gross deferred income tax assets													(143,029)	(109,203)
Less valuation allowance													69	70
Net deferred income tax assets													(142,960)	(109,133)
Deferred income taxes													383,813	360,785
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					2,528				2,365	$ 2,528	$ 2,365	$ 2,363
Additions based on tax positions related to current year										452	488	610
Additions based on tax positions related to prior year										39	520	618
Reductions based on tax positions related to prior years										(532)	(845)	(1,226)
Unrecognized tax benefits, ending balance		2,487				2,528				2,487	2,528	2,365
Interest on unrecognized tax benefits [Roll Forward]
Accrued interest on unrecognized tax benefits, beginning balance					311				92	311	92	101
Reduction in interest expense on uncertain tax positions										(27)	(102)	(97)
Interest expense on uncertain tax positions										104	321	88
Accrued interest on unrecognized tax benefits, ending balance		388				311				388	311	92
Unrecognized Tax Benefits
Unrecognized tax benefits		$ 2,487			$ 2,528	$ 2,528			$ 2,365	2,528	2,528	2,365	2,487	2,528	$ 2,365
Unrecognized tax benefits permanent differences													100	0	$ 0
MGE [Member]
Income tax provision components [Abstract]
Current payable: Federal										17,521	19,295	637
Current payable: State										3,497	3,443	(451)
Net-deferred: Federal										16,391	13,538	38,553
Net-deferred: State										4,485	5,305	10,625
Amortized investment tax credits										(103)	(175)	(219)
Total income tax provision										$ 41,791	$ 41,406	$ 49,145
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%	5.20%	5.10%
Amortized investment tax credits										(0.10%)	(0.20%)	(0.20%)
Credit for electricity from wind energy										(1.70%)	(1.80%)	(1.70%)
Domestic manufacturing deduction										(1.30%)	(1.40%)	0.00%
AFUDC equity, net										(0.20%)	(0.10%)	(0.80%)
Other, net, individually significant										(0.90%)	0.00%	0.10%
Effective income tax rate										35.90%	36.70%	37.50%
Deferred tax liabilities (assets) on balance sheet [Abstract]
Property-related													344,630	327,822
Investment in ATC	[1]												0	30,382
Bond transactions													1,324	1,422
Pension and other postretirement benefits													63,516	57,697
Derivatives													20,304	21,660
Tax deductible prepayments													8,404	8,011
Other deferred tax liabilities													16,571	14,831
Gross deferred income tax liabilities													454,749	461,825
Investment in ATC	[1]												0	0
Accrued expenses													(22,410)	(21,391)
Pension and other postretirement benefits													(48,860)	(46,582)
Deferred tax regulatory account													(903)	(1,047)
Derivatives													(20,304)	(21,660)
Other deferred tax assets													(19,224)	(18,589)
Gross deferred income tax assets													(111,701)	(109,269)
Less valuation allowance													69	70
Net deferred income tax assets													(111,632)	(109,199)
Deferred income taxes													343,117	$ 352,626
Operating loss deductions and tax credit carryforwards [Abstract]
State net tax operating loss deductions													$ 1,400

[1] As of December 1, 2016, MGE transferred its ownership interest in ATC to MGE Energy, resulting in a deferred intercompany gain and a corresponding step-up in tax basis.