Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Segment Reporting Information [Line Items]
Operating revenues	$ 137,838	$ 133,572	$ 404,318	$ 415,436
Interdepartmental revenues	0	0	0	0
Total operating revenues	137,838	133,572	404,318	415,436
Depreciation and amortization	(9,706)	(10,296)	(29,042)	(30,669)
Other operating expenses	(88,244)	(86,942)	(281,863)	(294,338)
Operating Income	39,888	36,334	93,413	90,429
Other (deductions) income, net	2,856	2,341	7,975	6,880
Interest (expense) income, net	(4,849)	(5,142)	(14,717)	(15,047)
Income before income taxes	37,895	33,533	86,671	82,262
Income tax provision	(14,253)	(12,495)	(32,711)	(30,718)
Net Income (Loss)	23,642	21,038	53,960	51,544
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,564,656		1,564,656		1,458,882
Capital Expenditures			65,909		65,176
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	118,914	114,963	305,189	291,804
Interdepartmental revenues	159	163	387	391
Total operating revenues	119,073	115,126	305,576	292,195
Depreciation and amortization	(6,530)	(7,144)	(19,554)	(21,173)
Other operating expenses	(78,823)	(78,118)	(223,876)	(218,556)
Operating Income	33,720	29,864	62,146	52,466
Other (deductions) income, net	342	35	602	68
Interest (expense) income, net	(2,535)	(2,747)	(7,727)	(8,100)
Income before income taxes	31,527	27,152	55,021	44,434
Income tax provision	(11,873)	(9,944)	(20,300)	(15,666)
Net Income (Loss)	19,654	17,208	34,721	28,768
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	875,477		875,477		794,738
Capital Expenditures			55,518		47,206
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	16,587	17,249	92,235	119,748
Interdepartmental revenues	4,658	5,083	10,721	8,247
Total operating revenues	21,245	22,332	102,956	127,995
Depreciation and amortization	(1,399)	(1,377)	(4,158)	(4,246)
Other operating expenses	(22,675)	(23,422)	(94,162)	(112,192)
Operating Income	(2,829)	(2,467)	4,636	11,557
Other (deductions) income, net	97	12	170	21
Interest (expense) income, net	(715)	(775)	(2,179)	(2,284)
Income before income taxes	(3,447)	(3,230)	2,627	9,294
Income tax provision	1,565	1,350	(740)	(3,532)
Net Income (Loss)	(1,882)	(1,880)	1,887	5,762
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	265,332		265,332		285,702
Capital Expenditures			9,749		15,830
Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	2,337	1,360	6,894	3,884
Interdepartmental revenues	8,679	9,582	26,032	28,559
Total operating revenues	11,016	10,942	32,926	32,443
Depreciation and amortization	(1,777)	(1,775)	(5,330)	(5,250)
Other operating expenses	(36)	(44)	(85)	(138)
Operating Income	9,203	9,123	27,511	27,055
Other (deductions) income, net	0	0	0	0
Interest (expense) income, net	(1,631)	(1,660)	(4,915)	(4,677)
Income before income taxes	7,572	7,463	22,596	22,378
Income tax provision	(3,039)	(2,994)	(9,069)	(8,981)
Net Income (Loss)	4,533	4,469	13,527	13,397
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	291,409		291,409		299,421
Capital Expenditures			642		2,140
Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Interdepartmental revenues	0	0	0	0
Total operating revenues	0	0	0	0
Depreciation and amortization	0	0	0	0
Other operating expenses	0	0	0	0
Operating Income	0	0	0	0
Other (deductions) income, net	2,280	2,167	6,764	6,434
Interest (expense) income, net	0	0	0	0
Income before income taxes	2,280	2,167	6,764	6,434
Income tax provision	(909)	(870)	(2,719)	(2,592)
Net Income (Loss)	1,371	1,297	4,045	3,842
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	60,236		60,236		57,006
Capital Expenditures			0		0
All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Interdepartmental revenues	0	0	0	0
Total operating revenues	0	0	0	0
Depreciation and amortization	0	0	0	0
Other operating expenses	(206)	(186)	(880)	(649)
Operating Income	(206)	(186)	(880)	(649)
Other (deductions) income, net	137	127	439	357
Interest (expense) income, net	32	40	104	14
Income before income taxes	(37)	(19)	(337)	(278)
Income tax provision	3	(37)	117	53
Net Income (Loss)	(34)	(56)	(220)	(225)
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	403,261		403,261		401,862
Capital Expenditures			0		0
Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Interdepartmental revenues	(13,496)	(14,828)	(37,140)	(37,197)
Total operating revenues	(13,496)	(14,828)	(37,140)	(37,197)
Depreciation and amortization	0	0	0	0
Other operating expenses	13,496	14,828	37,140	37,197
Operating Income	0	0	0	0
Other (deductions) income, net	0	0	0	0
Interest (expense) income, net	0	0	0	0
Income before income taxes	0	0	0	0
Income tax provision	0	0	0	0
Net Income (Loss)	0	0	0	0
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	(365,985)		(365,985)		(412,729)
Capital Expenditures			0		0
Assets Not Allocated [Member]
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	34,926		34,926		32,882
Capital Expenditures			0		0
MGE [Member]
Segment Reporting Information [Line Items]
Operating revenues	137,838	133,572	404,318	415,436
Interdepartmental revenues	0	0	0	0
Total operating revenues	137,838	133,572	404,318	415,436
Depreciation and amortization	(9,706)	(10,296)	(29,042)	(30,669)
Other operating expenses	(101,705)	(98,281)	(311,255)	(321,768)
Operating Income	26,427	24,995	64,021	62,999
Other (deductions) income, net	2,130	1,279	4,980	3,832
Interest (expense) income, net	(4,881)	(5,182)	(14,821)	(15,061)
Income tax provision	(13,669)	(11,525)	(30,273)	(28,082)
Net Income Including Noncontrolling Interest	23,676	21,092	54,180	51,770
Less Net Income Attributable to Noncontrolling Interest, net of tax	(6,153)	(6,010)	(18,305)	(17,935)
Net Income (Loss)	17,523	15,082	35,875	33,835
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,538,063		1,538,063		1,433,793
Capital Expenditures			65,909		65,176
MGE [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	118,914	114,963	305,189	291,804
Interdepartmental revenues	159	163	387	391
Total operating revenues	119,073	115,126	305,576	292,195
Depreciation and amortization	(6,530)	(7,144)	(19,554)	(21,173)
Other operating expenses	(90,946)	(88,013)	(244,303)	(234,145)
Operating Income	21,597	19,969	41,719	36,877
Other (deductions) income, net	592	(14)	729	(8)
Interest (expense) income, net	(2,535)	(2,747)	(7,727)	(8,100)
Net Income Including Noncontrolling Interest	19,654	17,208	34,721	28,769
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0	0	0
Net Income (Loss)	19,654	17,208	34,721	28,769
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	875,477		875,477		794,738
Capital Expenditures			55,518		47,206
MGE [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	16,587	17,249	92,235	119,748
Interdepartmental revenues	4,658	5,083	10,721	8,247
Total operating revenues	21,245	22,332	102,956	127,995
Depreciation and amortization	(1,399)	(1,377)	(4,158)	(4,246)
Other operating expenses	(21,180)	(22,057)	(94,938)	(115,701)
Operating Income	(1,334)	(1,102)	3,860	8,048
Other (deductions) income, net	167	(4)	206	(2)
Interest (expense) income, net	(715)	(775)	(2,179)	(2,284)
Net Income Including Noncontrolling Interest	(1,882)	(1,881)	1,887	5,762
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0	0	0
Net Income (Loss)	(1,882)	(1,881)	1,887	5,762
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	265,332		265,332		285,702
Capital Expenditures			9,749		15,830
MGE [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	2,337	1,360	6,894	3,884
Interdepartmental revenues	8,679	9,582	26,032	28,559
Total operating revenues	11,016	10,942	32,926	32,443
Depreciation and amortization	(1,777)	(1,775)	(5,330)	(5,250)
Other operating expenses	(3,075)	(3,039)	(9,154)	(9,119)
Operating Income	6,164	6,128	18,442	18,074
Other (deductions) income, net	0	0	0	0
Interest (expense) income, net	(1,631)	(1,660)	(4,915)	(4,677)
Net Income Including Noncontrolling Interest	4,533	4,468	13,527	13,397
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0	0	0
Net Income (Loss)	4,533	4,468	13,527	13,397
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	291,359		291,359		299,171
Capital Expenditures			642		2,140
MGE [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Interdepartmental revenues	0	0	0	0
Total operating revenues	0	0	0	0
Depreciation and amortization	0	0	0	0
Other operating expenses	0	0	0	0
Operating Income	0	0	0	0
Other (deductions) income, net	1,371	1,297	4,045	3,842
Interest (expense) income, net	0	0	0	0
Net Income Including Noncontrolling Interest	1,371	1,297	4,045	3,842
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0	0	0
Net Income (Loss)	1,371	1,297	4,045	3,842
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	60,236		60,236		57,006
Capital Expenditures			0		0
MGE [Member] | Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Interdepartmental revenues	(13,496)	(14,828)	(37,140)	(37,197)
Total operating revenues	(13,496)	(14,828)	(37,140)	(37,197)
Depreciation and amortization	0	0	0	0
Other operating expenses	13,496	14,828	37,140	37,197
Operating Income	0	0	0	0
Other (deductions) income, net	0	0	0	0
Interest (expense) income, net	0	0	0	0
Net Income Including Noncontrolling Interest	0	0	0	0
Less Net Income Attributable to Noncontrolling Interest, net of tax	(6,153)	(6,010)	(18,305)	(17,935)
Net Income (Loss)	(6,153)	(6,010)	(18,305)	(17,935)
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	10,740		10,740		(35,706)
Capital Expenditures			0		0
MGE [Member] | Assets Not Allocated [Member]
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	34,919		34,919		32,882
Capital Expenditures			$ 0		$ 0