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Income Taxes (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Income tax provision components [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Current payable: Federal |  |  |  |  |  |  |  |  | $ 8,017 |  | $ 18,622 |  | $ 21,125 |  |  |  |
Current payable: State |  |  |  |  |  |  |  |  | 4,647 |  | 5,163 |  | 5,129 |  |  |  |
Net-deferred: Federal |  |  |  |  |  |  |  |  | 3,510 |  | 120 |  | (8,346) |  |  |  |
Net-deferred: State |  |  |  |  |  |  |  |  | 3,702 |  | 3,629 |  | 4,264 |  |  |  |
Amortized investment tax credits |  |  |  |  |  |  |  |  | (92) |  | (100) |  | (78) |  |  |  |
Total income tax provision | $ 3,116 | $ 7,454 | $ 3,505 | $ 5,709 | $ 5,642 | $ 9,597 | $ 5,828 | $ 6,367 | $ 19,784 |  | $ 27,434 |  | $ 22,094 | [1] |  |  |
Reconciliation of tax provision to statutory federal income tax rate [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Statutory federal income tax rate |  |  |  |  |  |  |  |  | 21.00% |  | 21.00% |  | 35.00% |  |  |  |
State income taxes, net of federal benefit |  |  |  |  |  |  |  |  | 6.20% |  | 6.30% |  | 5.10% |  |  |  |
Amortized investment tax credits |  |  |  |  |  |  |  |  | (0.10%) |  | (0.10%) |  | 0.00% |  |  |  |
Credit for electricity from wind energy |  |  |  |  |  |  |  |  | (5.70%) | [2] | (0.30%) |  | (1.60%) |  |  |  |
Domestic manufacturing deduction |  |  |  |  |  |  |  |  | 0.00% |  | 0.00% |  | (1.40%) |  |  |  |
AFUDC equity, net |  |  |  |  |  |  |  |  | (0.30%) |  | (0.60%) |  | (0.20%) |  |  |  |
Federal income tax rate reduction |  |  |  |  |  |  |  |  | 0.00% |  | 0.00% |  | (18.10%) |  |  |  |
Amortization of utility excess deferred tax - tax reform |  |  |  |  |  |  |  |  | (2.40%) | [3] | (1.80%) | [3] | 0.00% |  |  |  |
Other, net, individually significant |  |  |  |  |  |  |  |  | (0.10%) |  | 0.10% |  | (0.30%) |  |  |  |
Effective income tax rate |  |  |  |  |  |  |  |  | 18.60% |  | 24.60% |  | 18.50% |  |  |  |
Components Of Deferred Tax Assets [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in ATC |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 22,576 | $ 23,638 |
Accrued expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 15,901 | 15,385 |
Pension and other postretirement benefits |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 30,968 | 34,914 |
Deferred tax regulatory account |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 35,493 | 37,121 |
Derivatives |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7,351 | 8,861 |
Leases |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,167 | 0 |
Other deferred tax assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12,540 | 15,137 |
Gross deferred income tax assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 129,996 | 135,056 |
Less valuation allowance |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (86) | (86) |
Net deferred income tax assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 129,910 | 134,970 |
Components Of Deferred Tax Liabilities [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property-related |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 245,083 | 244,114 |
Investment in ATC |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 51,569 | 50,771 |
Bond transactions |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 657 | 765 |
Pension and other postretirement benefits |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 45,683 | 47,644 |
Derivatives |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7,351 | 8,861 |
Tax deductible prepayments |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9,078 | 6,014 |
Leases |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,167 | 0 |
Other deferred tax liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8,624 | 8,753 |
Gross deferred income tax liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 373,212 | 366,922 |
Deferred income taxes |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 243,302 | 231,952 |
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 |  |  |  | 1,949 |  |  |  | 1,924 | $ 1,949 |  | $ 1,924 |  | $ 2,487 |  |  |  |
Additions based on tax positions related to current year |  |  |  |  |  |  |  |  | 741 |  | 425 |  | 552 |  |  |  |
Additions based on tax positions related to prior year |  |  |  |  |  |  |  |  | 84 |  | 272 |  | 19 |  |  |  |
Reductions based on tax positions related to prior years |  |  |  |  |  |  |  |  | (681) |  | (672) |  | (1,134) |  |  |  |
Unrecognized tax benefits, ending balance | 2,093 |  |  |  | 1,949 |  |  |  | 2,093 |  | 1,949 |  | 1,924 |  |  |  |
Interest on unrecognized tax benefits [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accrued interest on unrecognized tax benefits, beginning balance |  |  |  | 191 |  |  |  | 165 | 191 |  | 165 |  | 388 |  |  |  |
Reduction in interest expense on uncertain tax positions |  |  |  |  |  |  |  |  | (137) |  | (136) |  | (312) |  |  |  |
Interest expense on uncertain tax positions |  |  |  |  |  |  |  |  | 122 |  | 162 |  | 89 |  |  |  |
Accrued interest on unrecognized tax benefits, ending balance | 176 |  |  |  | 191 |  |  |  | 176 |  | 191 |  | 165 |  |  |  |
Unrecognized Tax Benefits |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrecognized tax benefits | $ 2,093 |  |  | $ 1,949 | $ 1,949 |  |  | $ 1,924 | 2,093 |  | 1,924 |  | 1,924 |  | 2,093 | 1,949 | $ 1,924
Unrecognized tax benefits permanent differences |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100 | 300 | $ 100
Excess Deferred Taxes, Tax Cuts And Jobs Act [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Effects Of The Tax Cuts And Jobs Act [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Excess Deferred Tax Income Statement Effect |  |  |  |  |  |  |  |  | 2,600 |  | 2,100 |  |  |  |  |  |
MGE [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income tax provision components [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Current payable: Federal |  |  |  |  |  |  |  |  | 7,616 |  | 19,926 |  | 21,512 |  |  |  |
Current payable: State |  |  |  |  |  |  |  |  | 4,608 |  | 5,704 |  | 5,316 |  |  |  |
Net-deferred: Federal |  |  |  |  |  |  |  |  | 2,242 |  | (2,563) |  | (11,195) |  |  |  |
Net-deferred: State |  |  |  |  |  |  |  |  | 3,044 |  | 2,494 |  | 3,435 |  |  |  |
Amortized investment tax credits |  |  |  |  |  |  |  |  | (92) |  | (100) |  | (78) |  |  |  |
Total income tax provision |  |  |  |  |  |  |  |  | $ 17,418 |  | $ 25,461 |  | $ 18,990 | [4] |  |  |
Reconciliation of tax provision to statutory federal income tax rate [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Statutory federal income tax rate |  |  |  |  |  |  |  |  | 21.00% |  | 21.00% |  | 35.00% |  |  |  |
State income taxes, net of federal benefit |  |  |  |  |  |  |  |  | 6.10% |  | 6.20% |  | 5.10% |  |  |  |
Amortized investment tax credits |  |  |  |  |  |  |  |  | (0.10%) |  | (0.10%) |  | 0.00% |  |  |  |
Credit for electricity from wind energy |  |  |  |  |  |  |  |  | (6.20%) | [2] | (0.40%) |  | (1.70%) |  |  |  |
Domestic manufacturing deduction |  |  |  |  |  |  |  |  | 0.00% |  | 0.00% |  | (1.50%) |  |  |  |
AFUDC equity, net |  |  |  |  |  |  |  |  | (0.30%) |  | (0.50%) |  | (0.20%) |  |  |  |
Federal income tax rate reduction |  |  |  |  |  |  |  |  | 0.00% |  | 0.00% |  | (19.30%) |  |  |  |
Amortization of utility excess deferred tax - tax reform |  |  |  |  |  |  |  |  | (2.70%) | [3] | (2.00%) | [3] | 0.00% |  |  |  |
Other, net, individually significant |  |  |  |  |  |  |  |  | (0.10%) |  | 0.20% |  | (0.40%) |  |  |  |
Effective income tax rate |  |  |  |  |  |  |  |  | 17.70% |  | 24.40% |  | 17.00% |  |  |  |
Components Of Deferred Tax Assets [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in ATC |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | 0 |
Accrued expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 15,890 | 15,385 |
Pension and other postretirement benefits |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 30,968 | 34,914 |
Deferred tax regulatory account |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 35,493 | 37,121 |
Derivatives |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7,351 | 8,861 |
Leases |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,167 | 0 |
Other deferred tax assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12,593 | 15,185 |
Gross deferred income tax assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 107,462 | 111,466 |
Less valuation allowance |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (86) | (86) |
Net deferred income tax assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 107,376 | 111,380 |
Components Of Deferred Tax Liabilities [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property-related |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 245,083 | 244,114 |
Investment in ATC |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | 0 |
Bond transactions |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 657 | 765 |
Pension and other postretirement benefits |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 45,683 | 47,644 |
Derivatives |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7,351 | 8,861 |
Tax deductible prepayments |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9,051 | 6,014 |
Leases |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,167 | 0 |
Other deferred tax liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8,425 | 8,598 |
Gross deferred income tax liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 321,417 | 315,996 |
Deferred income taxes |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 214,041 | $ 204,616 |
Operating loss deductions and tax credit carryforwards [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
State net tax operating loss deductions |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,400 |  |
MGE [Member] | Excess Deferred Taxes, Tax Cuts And Jobs Act [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Effects Of The Tax Cuts And Jobs Act [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Excess Deferred Tax Income Statement Effect |  |  |  |  |  |  |  |  | $ 2,600 |  | $ 2,100 |  |  |  |  |  |
[1] | In December 2017, there was a $21.7 million one-time tax impact as a result of the Tax Act.
[2] | Saratoga became operational in February 2019.
[3] | Included are impacts of the Tax Cuts and Jobs Act for the regulated utility for excess deferred taxes recognized using a normalization method of accounting. For the years ended December 31, 2019 and 2018, MGE recognized $2.6 million and $2.1 million, respectively. The amount and timing of the cash impacts will depend on the period over which certain income tax benefits are provided to customers, as determined by the PSCW.
[4] | In December 2017, there was a $21.7 million one-time tax impact as a result of the Tax Act.

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