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Joint Plant Ownership (Details) | 12 Months Ended
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Dec. 31, 2019 USD ($) operatingunit Turbines MW | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Assets held for sale | $ 0 | $ 3,080,000 |
Allowance for Funds Used During Construction |  |  |
AFUDC equity recognized | 2,300,000 | 3,300,000 | $ 1,200,000
Other Regulatory Asset [Member] |  |  |
Regulatory Assets and Liabilities Disclosure [Abstract] |  |  |
Regulatory Assets | $ 340,000 | $ 2,013,000 |
Public Service Commission of Wisconsin [Member] |  |  |
Allowance for Funds Used During Construction |  |  |
Authorized AFUDC rate | 50.00% | 50.00% | 50.00%
Authorized AFUDC rate - Signficiant projects (100%) | 100.00% | 100.00% | 100.00%
MGE [Member] |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Assets held for sale | $ 0 | $ 3,080,000 |
Columbia Units [Member] |  |  |
Jointly Owned Plants, Net Plant Ownership [Abstract] |  |  |
Plant | 291,997,000 | 292,157,000 |
Accumulated depreciation | (105,778,000) | (94,766,000) |
Property, plant, and equipment, net | 186,219,000 | 197,391,000 |
Construction work in progress | 1,777,000 | 2,021,000 |
Total property, plant, and equipment | $ 187,996,000 | $ 199,412,000 |
Columbia Units [Member] | MGE [Member] |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Jointly owned utility plant, proportion of total net summer generating capability | 29.00% |  |
Jointly owned utility plant, plant capacity (in MW) | MW | 217 |  |
Jointly owned utility plant, ownership interest | 19.10% | 19.40% |
Assets held for sale | $ 0 | $ 3,100,000 |
Jointly owned utility plant, fuel, operating, and maintenance expense | 32,600,000 | 36,500,000 | $ 36,200,000
Elm Road Units [Member] | MGE [Member] |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Jointly owned utility plant, fuel, operating, and maintenance expense | $ 19,700,000 | 17,600,000 | 17,300,000
Elm Road Units [Member] | MGE Power Elm Road [Member] |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Jointly owned utility plant, proportion of total net summer generating capability | 14.00% |  |
Jointly owned utility plant, proportion of total net summer generating capability, plant capacity (in MW) | MW | 105 |  |
Jointly owned utility plant, number of operating units | operatingunit | 2 |  |
Jointly Owned Plants, Net Plant Ownership [Abstract] |  |  |
Plant | $ 207,834,000 | 207,361,000 |
Accumulated depreciation | (37,071,000) | (33,897,000) |
Property, plant, and equipment, net | 170,763,000 | 173,464,000 |
Construction work in progress | 468,000 | 530,000 |
Total property, plant, and equipment | $ 171,231,000 | 173,994,000 |
Elm Road Unit 1 [Member] | MGE Power Elm Road [Member] |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Jointly owned utility plant, plant capacity (in MW) | MW | 615 |  |
Jointly owned utility plant, ownership interest | 8.33% |  |
Elm Road Unit 2 [Member] | MGE Power Elm Road [Member] |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Jointly owned utility plant, plant capacity (in MW) | MW | 615 |  |
Jointly owned utility plant, ownership interest | 8.33% |  |
Forward Wind [Member] |  |  |
Jointly Owned Plants, Net Plant Ownership [Abstract] |  |  |
Plant | $ 34,054,000 | 33,929,000 |
Accumulated depreciation | (13,413,000) | (12,530,000) |
Property, plant, and equipment, net | 20,641,000 | 21,399,000 |
Construction work in progress | 45,000 | 67,000 |
Total property, plant, and equipment | $ 20,686,000 | 21,466,000 |
Forward Wind [Member] | MGE [Member] |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Jointly owned utility plant, plant capacity (in MW) | MW | 18 |  |
Jointly owned utility plant, ownership interest | 12.80% |  |
Jointly owned utility plant, fuel, operating, and maintenance expense | $ 600,000 |  |
Forward Wind Turbines | Turbines | 86 |  |
Forward Wind Proportionate Purchase Price | $ 23,000,000 |  |
Forward Wind Purchase Power Energy Output | 12.80% |  |
Forward Wind [Member] | MGE [Member] | Other Regulatory Asset [Member] |  |  |
Regulatory Assets and Liabilities Disclosure [Abstract] |  |  |
Regulatory Assets |  | 600,000 |
Forward Wind [Member] | Joint Owners [Member] |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Forward Wind Aggregate Purchase Price | $ 174,000,000 |  |
West Campus [Member] | UW [Member] |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Jointly owned utility plant, ownership interest | 45.00% |  |
Jointly owned utility plant, fuel, operating, and maintenance expense | $ 6,600,000 | 6,300,000 | $ 5,300,000
West Campus [Member] | MGE Power West Campus [Member] |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Jointly owned utility plant, ownership interest | 55.00% |  |
Jointly Owned Plants, Net Plant Ownership [Abstract] |  |  |
Plant | $ 113,259,000 | 113,328,000 |
Accumulated depreciation | (33,786,000) | (32,055,000) |
Property, plant, and equipment, net | 79,473,000 | 81,273,000 |
Construction work in progress | 309,000 | 478,000 |
Total property, plant, and equipment | 79,782,000 | $ 81,751,000 |
Two Creeks [Member] |  |  |
Jointly Owned Plants, Net Plant Ownership [Abstract] |  |  |
Construction work in progress | 43,900,000 |  |
Projected Solar Farm Project Costs | 65,000,000 |  |
Allowance for Funds Used During Construction |  |  |
AFUDC equity recognized | $ 1,400,000 |  |
Two Creeks [Member] | MGE [Member] |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Jointly owned utility plant, plant capacity (in MW) | MW | 150 |  |
Jointly owned utility plant, ownership interest | 33.00% |  |
Two Creeks [Member] | MGE [Member] | Public Service Commission of Wisconsin [Member] |  |  |
Allowance for Funds Used During Construction |  |  |
Authorized AFUDC rate - Signficiant projects (100%) | 100.00% |  |
Badger Hollow I [Member] |  |  |
Jointly Owned Plants, Net Plant Ownership [Abstract] |  |  |
Construction work in progress | $ 18,700,000 |  |
Projected Solar Farm Project Costs | 65,000,000 |  |
Allowance for Funds Used During Construction |  |  |
AFUDC equity recognized | $ 300,000 |  |
Badger Hollow I [Member] | MGE [Member] |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Jointly owned utility plant, plant capacity (in MW) | MW | 150 |  |
Jointly owned utility plant, ownership interest | 33.00% |  |
Badger Hollow I [Member] | MGE [Member] | Public Service Commission of Wisconsin [Member] |  |  |
Allowance for Funds Used During Construction |  |  |
Authorized AFUDC rate - Signficiant projects (100%) | 100.00% |  |

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