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Taxes (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Reconciliation of tax provision to statutory federal income tax rate [Abstract] |  |  |  |  |
Statutory federal income tax rate |  | 21.00% | 21.00% | 21.00% | 21.00%
State income taxes, net of federal benefit |  | 6.20% | 6.30% | 6.20% | 6.30%
Amortized investment tax credits |  | (1.20%) | (0.10%) | (1.40%) | (0.10%)
Credit for electricity from wind energy |  | (4.80%) | (5.00%) | (6.00%) | (6.10%)
AFUDC equity, net |  | (1.10%) | (1.10%) | (1.00%) | (1.30%)
Amortization of utility excess deferred tax - tax reform | [1] | (12.40%) | (2.00%) | (14.50%) | (2.40%)
Other, net, individually significant |  | (0.40%) | (0.40%) | (0.10%) | (0.10%)
Effective income tax rate |  | 7.30% | 18.70% | 4.20% | 17.30%
Excess Deferred Taxes, Tax Cuts And Jobs Act [Member] |  |  |  |  |
Effects Of The Tax Cuts And Jobs Act [Abstract] |  |  |  |  |
Excess deferred tax - Income statement effect |  | $ 0.7 | $ 0.7 | $ 2.0 | $ 2.0
Return of unprotected excess deferred taxes |  | $ 3.3 |  | $ 9.9 |
MGE [Member] |  |  |  |  |
Reconciliation of tax provision to statutory federal income tax rate [Abstract] |  |  |  |  |
Statutory federal income tax rate |  | 21.00% | 21.00% | 21.00% | 21.00%
State income taxes, net of federal benefit |  | 6.20% | 6.20% | 6.20% | 6.20%
Amortized investment tax credits |  | (1.30%) | (0.10%) | (1.50%) | (0.10%)
Credit for electricity from wind energy |  | (5.20%) | (5.40%) | (6.60%) | (6.60%)
AFUDC equity, net |  | (1.20%) | (1.20%) | (1.10%) | (1.40%)
Amortization of utility excess deferred tax - tax reform | [1] | (13.50%) | (2.20%) | (15.90%) | (2.60%)
Other, net, individually significant |  | (0.30%) | (0.40%) | 0.00% | 0.00%
Effective income tax rate |  | 5.70% | 17.90% | 2.10% | 16.50%
[1] | Included are impacts of the 2017 Tax Act for the regulated utility for excess deferred taxes recognized using a normalization method of accounting in recognition of IRS rules that restrict the rate at which the excess deferred taxes may be returned to utility customers. For the three months ended September 30, 2021 and 2020, MGE recognized $0.7 million. For the nine months ended September 30, 2021 and 2020, MGE recognized $2.0 million. Included in the 2021 rate settlement was a one-time return to customers of the electric portion of excess deferred taxes related to the 2017 Tax Act not restricted by IRS normalization rules. For the three and nine months ended September 30, 2021, MGE recognized $3.3 million and $9.9 million, respectively.

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- Definition 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. The amount and timing of the cash impacts will depend on the period over which certain income tax benefits are provided to customers, determined by the PSCW. + References No definition available. + Details
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