Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Operating revenues	$ 140,099	$ 137,838	$ 435,624	$ 404,318
Interdepartmental revenues	0	0	0	0
Total operating revenues	140,099	137,838	435,624	404,318
Depreciation and amortization	(9,710)	(9,706)	(28,864)	(29,042)
Other operating expenses	(90,489)	(88,244)	(304,735)	(281,863)
Operating Income	39,900	39,888	102,025	93,413
Other income, net	3,750	2,856	9,770	7,975
Interest (expense) income, net	(4,605)	(4,849)	(13,937)	(14,717)
Income before income taxes	39,045	37,895	97,858	86,671
Income tax (provision) benefit	(14,692)	(14,253)	(37,030)	(32,711)
Net Income (Loss)	24,353	23,642	60,828	53,960
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,643,711		1,643,711		1,586,924
Capital Expenditures			92,092		98,435
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	119,836	118,914	310,176	305,189
Interdepartmental revenues	172	159	410	387
Total operating revenues	120,008	119,073	310,586	305,576
Depreciation and amortization	(6,425)	(6,530)	(19,139)	(19,554)
Other operating expenses	(82,414)	(78,823)	(235,390)	(223,876)
Operating Income	31,169	33,720	56,057	62,146
Other income, net	1,166	342	2,226	602
Interest (expense) income, net	(2,309)	(2,535)	(7,028)	(7,727)
Income before income taxes	30,026	31,527	51,255	55,021
Income tax (provision) benefit	(11,021)	(11,873)	(18,148)	(20,300)
Net Income (Loss)	19,005	19,654	33,107	34,721
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	948,030		948,030		888,444
Capital Expenditures			79,637		81,965
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	18,864	16,587	121,373	92,235
Interdepartmental revenues	2,757	4,658	10,449	10,721
Total operating revenues	21,621	21,245	131,822	102,956
Depreciation and amortization	(1,497)	(1,399)	(4,357)	(4,158)
Other operating expenses	(21,485)	(22,675)	(111,494)	(94,162)
Operating Income	(1,361)	(2,829)	15,971	4,636
Other income, net	92	97	69	170
Interest (expense) income, net	(729)	(715)	(2,166)	(2,179)
Income before income taxes	(1,998)	(3,447)	13,874	2,627
Income tax (provision) benefit	751	1,565	(5,844)	(740)
Net Income (Loss)	(1,247)	(1,882)	8,030	1,887
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	274,896		274,896		285,468
Capital Expenditures			9,423		13,812
Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	1,399	2,337	4,075	6,894
Interdepartmental revenues	10,660	8,679	31,924	26,032
Total operating revenues	12,059	11,016	35,999	32,926
Depreciation and amortization	(1,788)	(1,777)	(5,368)	(5,330)
Other operating expenses	(33)	(36)	(96)	(85)
Operating Income	10,238	9,203	30,535	27,511
Other income, net	0	0	0	0
Interest (expense) income, net	(1,596)	(1,631)	(4,814)	(4,915)
Income before income taxes	8,642	7,572	25,721	22,596
Income tax (provision) benefit	(3,469)	(3,039)	(10,209)	(9,069)
Net Income (Loss)	5,173	4,533	15,512	13,527
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	289,808		289,808		323,216
Capital Expenditures			3,032		2,658
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	(1)	0	(1)	0
Operating Income	(1)	0	(1)	0
Other income, net	2,340	2,280	7,045	6,764
Interest (expense) income, net	0	0	0	0
Income before income taxes	2,339	2,280	7,044	6,764
Income tax (provision) benefit	(939)	(909)	(2,837)	(2,719)
Net Income (Loss)	1,400	1,371	4,207	4,045
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	63,666		63,666		61,064
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	(145)	(206)	(537)	(880)
Operating Income	(145)	(206)	(537)	(880)
Other income, net	152	137	430	439
Interest (expense) income, net	29	32	71	104
Income before income taxes	36	(37)	(36)	(337)
Income tax (provision) benefit	(14)	3	8	117
Net Income (Loss)	22	(34)	(28)	(220)
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	408,340		408,340		413,291
Capital Expenditures			0		0
Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Interdepartmental revenues	(13,589)	(13,496)	(42,783)	(37,140)
Total operating revenues	(13,589)	(13,496)	(42,783)	(37,140)
Depreciation and amortization	0	0	0	0
Other operating expenses	13,589	13,496	42,783	37,140
Operating Income	0	0	0	0
Other 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) benefit	0	0	0	0
Net Income (Loss)	0	0	0	0
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	(391,992)		(391,992)		(403,118)
Capital Expenditures			0		0
Assets Not Allocated [Member]
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	50,963		50,963		18,559
Capital Expenditures			0		0
MGE [Member]
Segment Reporting Information [Line Items]
Operating revenues	140,099	137,838	435,624	404,318
Interdepartmental revenues	0	0	0	0
Total operating revenues	140,099	137,838	435,624	404,318
Depreciation and amortization	(9,710)	(9,706)	(28,864)	(29,042)
Other operating expenses	(104,143)	(101,705)	(338,435)	(311,255)
Operating Income	26,246	26,427	68,325	64,021
Other income, net	2,719	2,130	6,539	4,980
Interest (expense) income, net	(4,634)	(4,881)	(14,008)	(14,821)
Income tax (provision) benefit	(13,798)	(13,669)	(34,237)	(30,273)
Net Income Including Noncontrolling Interest	24,331	23,676	60,856	54,180
Less Net Income Attributable to Noncontrolling Interest, net of tax	(6,861)	(6,153)	(20,547)	(18,305)
Net Income (Loss)	17,470	17,523	40,309	35,875
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,617,432		1,617,432		1,553,651
Capital Expenditures			92,092		98,435
MGE [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	119,836	118,914	310,176	305,189
Interdepartmental revenues	172	159	410	387
Total operating revenues	120,008	119,073	310,586	305,576
Depreciation and amortization	(6,425)	(6,530)	(19,139)	(19,554)
Other operating expenses	(93,481)	(90,946)	(253,566)	(244,303)
Operating Income	20,102	21,597	37,881	41,719
Other income, net	1,212	592	2,254	729
Interest (expense) income, net	(2,309)	(2,535)	(7,028)	(7,727)
Net Income Including Noncontrolling Interest	19,005	19,654	33,107	34,721
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0	0	0
Net Income (Loss)	19,005	19,654	33,107	34,721
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	948,030		948,030		888,444
Capital Expenditures			79,637		81,965
MGE [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	18,864	16,587	121,373	92,235
Interdepartmental revenues	2,757	4,658	10,449	10,721
Total operating revenues	21,621	21,245	131,822	102,956
Depreciation and amortization	(1,497)	(1,399)	(4,357)	(4,158)
Other operating expenses	(20,748)	(21,180)	(117,346)	(94,938)
Operating Income	(624)	(1,334)	10,119	3,860
Other income, net	106	167	77	206
Interest (expense) income, net	(729)	(715)	(2,166)	(2,179)
Net Income Including Noncontrolling Interest	(1,247)	(1,882)	8,030	1,887
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0	0	0
Net Income (Loss)	(1,247)	(1,882)	8,030	1,887
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	274,896		274,896		285,468
Capital Expenditures			9,423		13,812
MGE [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	1,399	2,337	4,075	6,894
Interdepartmental revenues	10,660	8,679	31,924	26,032
Total operating revenues	12,059	11,016	35,999	32,926
Depreciation and amortization	(1,788)	(1,777)	(5,368)	(5,330)
Other operating expenses	(3,502)	(3,075)	(10,305)	(9,154)
Operating Income	6,769	6,164	20,326	18,442
Other income, net	0	0	0	0
Interest (expense) income, net	(1,596)	(1,631)	(4,814)	(4,915)
Net Income Including Noncontrolling Interest	5,173	4,533	15,512	13,527
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0	0	0
Net Income (Loss)	5,173	4,533	15,512	13,527
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	289,758		289,758		323,166
Capital Expenditures			3,032		2,658
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	(1)	0	(1)	0
Operating Income	(1)	0	(1)	0
Other income, net	1,401	1,371	4,208	4,045
Interest (expense) income, net	0	0	0	0
Net Income Including Noncontrolling Interest	1,400	1,371	4,207	4,045
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0	0	0
Net Income (Loss)	1,400	1,371	4,207	4,045
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	63,666		63,666		61,064
Capital Expenditures			0		0
MGE [Member] | Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Interdepartmental revenues	(13,589)	(13,496)	(42,783)	(37,140)
Total operating revenues	(13,589)	(13,496)	(42,783)	(37,140)
Depreciation and amortization	0	0	0	0
Other operating expenses	13,589	13,496	42,783	37,140
Operating Income	0	0	0	0
Other 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	(6,861)	(6,153)	(20,547)	(18,305)
Net Income (Loss)	(6,861)	(6,153)	(20,547)	(18,305)
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	(9,881)		(9,881)		(23,050)
Capital Expenditures			0		0
MGE [Member] | Assets Not Allocated [Member]
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	50,963		50,963		18,559
Capital Expenditures			$ 0		$ 0