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Segment Information (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Segment Reporting Information [Line Items]                      
Operating revenues $ 140,274 $ 139,539 $ 126,463 $ 156,823 $ 138,925 $ 136,717 $ 121,576 $ 147,527 $ 563,099 $ 544,745 $ 564,028
Depreciation and amortization                 (53,077) (44,646) (44,225)
Other operating expenses                 (381,286) (371,871) (395,548)
Operating Income (Loss) 26,274 43,001 26,551 32,910 23,958 46,527 28,212 29,531 128,736 128,228 124,255
Other (deductions) income, net                 10,288 9,711 8,613
Interest (expense) income, net                 (19,324) (19,866) (20,162)
Income before income taxes                 119,700 118,073 112,706
Income tax (provision) benefit 13,393 [1] (15,584) (8,736) (11,167) (7,570) (15,714) (9,284) (9,945) (22,094) [1] (42,513) (41,363)
Net Income 36,134 $ 26,629 $ 15,543 $ 19,300 14,502 $ 27,880 $ 16,150 $ 17,028 97,606 75,560 71,343
Segment Reporting Information Assets And Capital Expenditures [Abstract]                      
Assets 1,855,182       1,801,060       1,855,182 1,801,060 1,726,403
Capital expenditures                 108,131 83,659 72,030
Effects Of The Tax Cuts And Jobs Act [Abstract]                      
Investment in ATC 60,426       73,616       60,426 73,616  
Tax Cuts and Jobs Act [Member]                      
Effects Of The Tax Cuts And Jobs Act [Abstract]                      
Deferred Income Tax Expense Tax Cuts And Jobs Act                 (21,700)    
Electric [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 413,926 409,006 412,528
Gas [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 148,825 134,543 143,737
Non Regulated Energy [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 348 1,196 7,763
Transmission Investment [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 0 0 0
All Others [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 0 0 0
Operating Segments [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 563,099 544,745 564,028
Operating Segments [Member] | Electric [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 413,416 410,918 413,041
Depreciation and amortization                 (36,660) (29,122) (29,945)
Other operating expenses                 (305,539) (310,941) (318,001)
Operating Income (Loss)                 71,217 70,855 65,095
Other (deductions) income, net                 1,978 960 400
Interest (expense) income, net                 (11,257) (11,147) (11,187)
Income before income taxes                 61,938 60,668 54,308
Income tax (provision) benefit                 (20,547) [1] (20,115) (17,915)
Net Income                 41,391 40,553 36,393
Segment Reporting Information Assets And Capital Expenditures [Abstract]                      
Assets 1,043,181       1,021,905       1,043,181 1,021,905 974,235
Capital expenditures                 77,353 50,699 49,370
Operating Segments [Member] | Gas [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 163,799 155,921 155,517
Depreciation and amortization                 (9,000) (8,128) (6,758)
Other operating expenses                 (133,479) (127,083) (128,241)
Operating Income (Loss)                 21,320 20,710 20,518
Other (deductions) income, net                 362 30 (33)
Interest (expense) income, net                 (3,234) (3,223) (3,203)
Income before income taxes                 18,448 17,517 17,282
Income tax (provision) benefit                 (7,303) [1] (6,894) (6,915)
Net Income                 11,145 10,623 10,367
Segment Reporting Information Assets And Capital Expenditures [Abstract]                      
Assets 344,337       318,603       344,337 318,603 298,435
Capital expenditures                 26,847 29,136 18,787
Operating Segments [Member] | Non Regulated Energy [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 44,816 45,126 47,198
Depreciation and amortization                 (7,417) (7,372) (7,475)
Other operating expenses                 (190) (155) (158)
Operating Income (Loss)                 37,209 37,599 39,565
Other (deductions) income, net                 0 0 0
Interest (expense) income, net                 (5,533) (5,768) (5,993)
Income before income taxes                 31,676 31,831 33,572
Income tax (provision) benefit                 8,860 [1] (12,775) (13,474)
Net Income                 40,536 19,056 20,098
Segment Reporting Information Assets And Capital Expenditures [Abstract]                      
Assets 270,384       271,277       270,384 271,277 277,858
Capital expenditures                 3,931 3,824 3,873
Operating Segments [Member] | Transmission Investment [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 0 0 0
Depreciation and amortization                 0 0 0
Other operating expenses                 (9) (17) (19)
Operating Income (Loss)                 (9) (17) (19)
Other (deductions) income, net                 9,844 8,429 7,728
Interest (expense) income, net                 0 0 0
Income before income taxes                 9,835 8,412 7,709
Income tax (provision) benefit                 (3,954) (2,836) (3,102)
Net Income                 5,881 5,576 4,607
Segment Reporting Information Assets And Capital Expenditures [Abstract]                      
Assets 61,783 [2]       74,535       61,783 [2] 74,535 69,470
Capital expenditures                 0 0 0
Operating Segments [Member] | Transmission Investment [Member] | Tax Cuts and Jobs Act [Member]                      
Effects Of The Tax Cuts And Jobs Act [Abstract]                      
Investment in ATC (20,400)               (20,400)    
Operating Segments [Member] | All Others [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 0 0 0
Depreciation and amortization                 0 (24) (47)
Other operating expenses                 (1,001) (895) (857)
Operating Income (Loss)                 (1,001) (919) (904)
Other (deductions) income, net                 (1,896) 292 518
Interest (expense) income, net                 700 272 221
Income before income taxes                 (2,197) (355) (165)
Income tax (provision) benefit                 850 [1] 107 43
Net Income                 (1,347) (248) (122)
Segment Reporting Information Assets And Capital Expenditures [Abstract]                      
Assets 485,548       465,202       485,548 465,202 434,868
Capital expenditures                 0 0 0
Operating Segments [Member] | Assets Not Allocated [Member]                      
Segment Reporting Information Assets And Capital Expenditures [Abstract]                      
Assets 26,345       27,338       26,345 27,338 49,753
Capital expenditures                 0 0 0
Consolidation/Elimination Entries [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 (58,932) (67,220) (51,728)
Depreciation and amortization                 0 0 0
Other operating expenses                 58,932 67,220 51,728
Operating Income (Loss)                 0 0 0
Other (deductions) income, net                 0 0 0
Interest (expense) income, net                 0 0 0
Income before income taxes                 0 0 0
Income tax (provision) benefit                 0 0 0
Net Income                 0 0 0
Segment Reporting Information Assets And Capital Expenditures [Abstract]                      
Assets (376,396)       (377,800)       (376,396) (377,800) (378,216)
Capital expenditures                 0 0 0
Consolidation/Elimination Entries [Member] | Electric [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 (510) 1,912 513
Consolidation/Elimination Entries [Member] | Gas [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 14,974 21,378 11,780
Consolidation/Elimination Entries [Member] | Non Regulated Energy [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 44,468 43,930 39,435
Consolidation/Elimination Entries [Member] | Transmission Investment [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 0 0 0
Consolidation/Elimination Entries [Member] | All Others [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 0 0 0
MGE [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 563,111 544,798 564,065
Depreciation and amortization                 (53,077) (44,622) (44,178)
Other operating expenses [3]                 (398,567) [1] (410,627) (432,887)
Operating Income (Loss) [3]                 111,467 89,549 87,000
Other (deductions) income, net [3]                 1,629 5,181 4,848
Interest (expense) income, net                 (20,024) (20,138) (20,383)
Income tax (provision) benefit                 (18,990) (41,791) (41,406)
Net Income Including Noncontrolling Interest                 93,072 74,592 71,465
Less Net Income Attributable to Noncontrolling Interest, net of tax                 (43,237) (23,358) (26,097)
Net Income                 49,835 51,234 45,368
Segment Reporting Information Assets And Capital Expenditures [Abstract]                      
Assets 1,684,041       1,638,852       1,684,041 1,638,852 1,669,514
Capital expenditures                 108,131 83,659 72,030
Effects Of The Tax Cuts And Jobs Act [Abstract]                      
Investment in ATC 0       0       0 0  
MGE [Member] | Tax Cuts and Jobs Act [Member]                      
Effects Of The Tax Cuts And Jobs Act [Abstract]                      
Deferred Income Tax Expense Tax Cuts And Jobs Act                 (21,700)    
MGE [Member] | Electric [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 413,929 409,030 412,550
MGE [Member] | Gas [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 148,834 134,572 143,752
MGE [Member] | Non Regulated Energy [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 348 1,196 7,763
MGE [Member] | Transmission Investment [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 0 0 [4] 0
MGE [Member] | Operating Segments [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 563,111 544,798 564,065
MGE [Member] | Operating Segments [Member] | Electric [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 413,416 410,918 413,041
Depreciation and amortization                 (36,660) (29,122) (29,945)
Other operating expenses [3]                 (325,532) [1] (330,924) (335,803)
Operating Income (Loss) [3]                 51,224 50,872 47,293
Other (deductions) income, net [3]                 1,424 828 287
Interest (expense) income, net                 (11,257) (11,147) (11,187)
Net Income Including Noncontrolling Interest                 41,391 40,553 36,393
Less Net Income Attributable to Noncontrolling Interest, net of tax                 0 0 0
Net Income                 41,391 40,553 36,393
Segment Reporting Information Assets And Capital Expenditures [Abstract]                      
Assets 1,043,181       1,021,905       1,043,181 1,021,905 974,235
Capital expenditures                 77,353 50,699 49,370
MGE [Member] | Operating Segments [Member] | Gas [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 163,799 155,921 155,517
Depreciation and amortization                 (9,000) (8,128) (6,758)
Other operating expenses [3]                 (140,625) [1] (133,940) (135,124)
Operating Income (Loss) [3]                 14,174 13,853 13,635
Other (deductions) income, net [3]                 205 (7) (65)
Interest (expense) income, net                 (3,234) (3,223) (3,203)
Net Income Including Noncontrolling Interest                 11,145 10,623 10,367
Less Net Income Attributable to Noncontrolling Interest, net of tax                 0 0 0
Net Income                 11,145 10,623 10,367
Segment Reporting Information Assets And Capital Expenditures [Abstract]                      
Assets 344,337       318,603       344,337 318,603 298,435
Capital expenditures                 26,847 29,136 18,787
MGE [Member] | Operating Segments [Member] | Non Regulated Energy [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 44,816 45,126 47,198
Depreciation and amortization                 (7,417) (7,372) (7,475)
Other operating expenses [3]                 8,670 [1] (12,930) (13,632)
Operating Income (Loss) [3]                 46,069 24,824 26,091
Other (deductions) income, net [3]                 0 0 0
Interest (expense) income, net                 (5,533) (5,768) (5,993)
Net Income Including Noncontrolling Interest                 40,536 19,056 20,098
Less Net Income Attributable to Noncontrolling Interest, net of tax                 0 0 0
Net Income                 40,536 19,056 20,098
Segment Reporting Information Assets And Capital Expenditures [Abstract]                      
Assets 270,334       271,227       270,334 271,227 277,808
Capital expenditures                 3,931 3,824 3,873
MGE [Member] | Operating Segments [Member] | Transmission Investment [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 0 0 [4] 0
Depreciation and amortization                 0 0 [4] 0
Other operating expenses [3]                 0 0 [4] (19)
Operating Income (Loss) [3]                 0 0 [4] (19)
Other (deductions) income, net [3]                 0 4,360 [4] 4,626
Interest (expense) income, net                 0 0 [4] 0
Net Income Including Noncontrolling Interest                 0 4,360 [4] 4,607
Less Net Income Attributable to Noncontrolling Interest, net of tax                 0 0 [4] 0
Net Income                 0 4,360 [4] 4,607
Segment Reporting Information Assets And Capital Expenditures [Abstract]                      
Assets 0       0 [5]       0 0 [5] 69,470
Capital expenditures                 0 0 0
MGE [Member] | Operating Segments [Member] | Assets Not Allocated [Member]                      
Segment Reporting Information Assets And Capital Expenditures [Abstract]                      
Assets 26,345       27,338       26,345 27,338 49,753
Capital expenditures                 0 0 0
MGE [Member] | Consolidation/Elimination Entries [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 (58,920) (67,167) (51,691)
Depreciation and amortization                 0 0 0
Other operating expenses [3]                 58,920 67,167 51,691
Operating Income (Loss) [3]                 0 0 0
Other (deductions) income, net [3]                 0 0 0
Interest (expense) income, net                 0 0 0
Net Income Including Noncontrolling Interest                 0 0 0
Less Net Income Attributable to Noncontrolling Interest, net of tax                 (43,237) (23,358) (26,097)
Net Income                 (43,237) (23,358) (26,097)
Segment Reporting Information Assets And Capital Expenditures [Abstract]                      
Assets $ (156)       $ (221)       (156) (221) (187)
Capital expenditures                 0 0 0
MGE [Member] | Consolidation/Elimination Entries [Member] | Electric [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 (513) 1,888 491
MGE [Member] | Consolidation/Elimination Entries [Member] | Gas [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 14,965 21,349 11,765
MGE [Member] | Consolidation/Elimination Entries [Member] | Non Regulated Energy [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 44,468 43,930 39,435
MGE [Member] | Consolidation/Elimination Entries [Member] | Transmission Investment [Member]                      
Segment Reporting Information [Line Items]                      
Operating revenues                 $ 0 $ 0 [4] $ 0
[1]
In December 2017, there was a $21.7 million one-time tax impact as a result of the Tax Act. See Footnote 12 for further information.
[2]
In December 2017, there was a $20.4 million one-time tax impact as a result of the Tax Act. See Footnote 12 for further information.
[3]
Amounts are shown net of the related tax expense, consistent with the presentation on the MGE Consolidated Statements of Income.
[4]
As of July 31, 2016, MGE no longer consolidates MGE Energy's proportionate share of equity earnings in MGE Transco. See Footnote 4 for further information.
[5]
In December 2016, MGE’s ownership interest in MGE Transco was transferred to MGE Energy. See Footnote 4 for additional information.