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Significant Accounting Policies (Policies) | 6 Months Ended
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Jun. 30, 2026
Accounting Policies [Abstract] |
Variable Interest Entities | MGE Power Elm Road and MGE Power West Campus own electric generating assets and lease those assets to MGE. Both entities are variable interest entities (VIE) under applicable authoritative accounting guidance. MGE is considered the primary beneficiary of these entities as a result of contractual agreements. As a result, MGE has consolidated MGE Power Elm Road and MGE Power West Campus in its financial reports.
Cash and Cash Equivalents | Cash Equivalents All highly liquid investments purchased with an original maturity of three months or less are considered to be cash equivalents.
Restricted Cash | Restricted Cash MGE has certain cash accounts that are restricted to uses other than current operations and designated for a specific purpose. MGE's restricted cash accounts include cash held by trustees for certain employee benefits and cash deposits held by third parties. These are included in "Other current assets" on the consolidated balance sheets.
Receivable Margin Account | Receivable – Margin Account Cash amounts held by counterparties as margin collateral for certain financial transactions are recorded as Receivable – margin account in "Other current assets" on the consolidated balance sheets. The costs being hedged are fuel for electric generation, purchased power, and cost of gas sold.
New Accounting Pronouncement Disclosure | In November 2024, the Financial Accounting Standards Board issued authoritative guidance within the codification's Income Statement - Reporting Comprehensive Income topic, which added disclosure requirements for the disaggregation of certain income statement expenses. The authoritative guidance will become effective for annual reporting periods beginning after December 15, 2026, and interim periods beginning after December 15, 2027. MGE will adopt the standard as of the effective date. The adoption of this standard will not have a material impact on MGE Energy's and MGE's financial statements. In September 2025, the Financial Accounting Standards Board issued authoritative guidance within the codification’s Internal-Use Software topic, which amends certain aspects of the accounting for and disclosure of software costs. The authoritative guidance will become effective for annual reporting periods beginning after December 15, 2027, and interim periods within those annual reporting periods; early adoption is permitted as of the beginning of an annual reporting period. MGE will adopt the standard as of the effective date. The adoption of this standard is not expected to have a material impact on MGE Energy's and MGE’s financial statements. In May 2026, the Financial Accounting Standards Board issued authoritative guidance within the codification’s Environmental Credits topic, which establishes a comprehensive model for the recognition, measurement, presentation, and disclosure of environmental credits and related obligations. The authoritative guidance will become effective for annual reporting periods beginning after December 15, 2027, and interim periods within those annual reporting periods; early adoption is permitted. MGE will adopt the standard as of the effective date. The adoption of this standard is not expected to have a material impact on MGE Energy's and MGE’s financial statements.
Investments - ATC and ATC Holdco | MGE Transco and MGEE Transco have accounted for their investments in ATC and ATC Holdco, respectively, under the equity method of accounting.
Pension and Other Postretirement Benefit Plan Assets | MGE maintains qualified and nonqualified pension plans, health care, and life insurance benefits and defined contribution 401(k) benefit plans for its employees and retirees. The components of net periodic benefit cost, other than the service cost component, are recorded in "Other income, net" on the consolidated statements of income. The service cost component is recorded in "Other operations and maintenance" on the consolidated statements of income. MGE has regulatory treatment and recognizes regulatory assets or liabilities for timing differences between when net periodic benefit costs are recovered and when costs are recognized.
Common Stock | MGE Energy sells shares of its common stock through its Direct Stock Purchase and Dividend Reinvestment Plan (the Stock Plan). Those shares may be newly issued shares or shares that are purchased in the open market by an independent agent for participants in the Stock Plan. Sales of newly issued shares under the Stock Plan are covered by a shelf registration statement that MGE Energy filed with the SEC.
Earnings Per Share | Basic earnings per share is calculated by dividing net income attributable to MGE Energy by the weighted-average number of MGE Energy's common shares outstanding during the period. In the calculation of diluted earnings per share, weighted-average shares outstanding are increased for additional shares that would be outstanding if potentially dilutive securities were converted to common stock.
Wisconsin Fuel Rules | Fuel rules require Wisconsin utilities to defer electric fuel-related costs that fall outside a symmetrical cost tolerance band around the amount approved for a utility in its annual fuel proceedings. Any over- or under-recovery of the actual costs is determined in the following year and is then reflected in future billings to electric retail customers. The fuel rules bandwidth is set at plus or minus 2% in 2026 and 2025. The electric fuel-related costs are subject to an excess revenues test. Excess revenues are defined as revenues in the year in question that provide MGE with a greater return on common equity than authorized by the PSCW in MGE's latest rate order. The recovery of under-collected electric fuel-related costs would be reduced by the amount that exceeds the excess revenue test. These costs are subject to the PSCW's annual review of fuel costs completed in the year following the deferral.
Derivative Hedging | As part of its regular operations, MGE enters into contracts, including options, swaps, futures, forwards, and other contractual commitments, to manage its exposure to commodity prices. To the extent that these contracts are derivatives, MGE assesses whether or not the normal purchases or normal sales exclusion applies. For contracts to which this exclusion cannot be applied, the derivatives are recognized in the consolidated balance sheets at fair value. MGE's financial commodity derivative activities are conducted in accordance with its electric and gas risk management program, which is approved by the PSCW and limits the volume MGE can hedge with specific risk management strategies. The maximum length of time over which cash flows related to energy commodities can be hedged is four years. If the derivative qualifies for regulatory deferral, the derivatives are marked to fair value and are offset with a corresponding regulatory asset or liability depending on whether the derivative is in a net loss or net gain position, respectively. The deferred gain or loss is recognized in earnings in the delivery month applicable to the instrument. Gains and losses related to hedges qualifying for regulatory treatment are refundable or recoverable in gas rates through the Purchased Gas Adjustment (PGA) or in electric rates as a component of the fuel rules mechanism.
Derivative Netting | All derivative instruments in this table are presented on a gross basis and are calculated prior to the netting of instruments with the same counterparty under a master netting agreement as well as the netting of collateral. For financial statement purposes, instruments are netted with the same counterparty under a master netting agreement as well as the netting of collateral.
Recurring Fair Value Measurements | Fair value is defined as the price that would be received to sell an asset or would be paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants at the measurement date. The accounting standard clarifies that fair value should be based on the assumptions market participants would use when pricing the asset or liability including assumptions about risk. The standard also establishes a three-level fair value hierarchy based upon the observability of the assumptions used and requires the use of observable market data when available. The levels are: Level 1 - Pricing inputs are quoted prices within active markets for identical assets or liabilities. Level 2 - Pricing inputs are quoted prices within active markets for similar assets or liabilities; quoted prices for identical or similar instruments in markets that are not active; and model-derived valuations that are correlated with or otherwise verifiable by observable market data. Level 3 - Pricing inputs are unobservable and reflect management's best estimate of what market participants would use in pricing the asset or liability. a. Fair Value of Financial Assets and Liabilities Recorded at the Carrying Amount. The carrying amount of cash, cash equivalents, and outstanding commercial paper approximates fair market value due to the short maturity of those investments and obligations. The estimated fair market value of long-term debt is based on quoted market prices for similar financial instruments. Since long-term debt is not traded in an active market, it is classified as Level 2.Investments include exchange-traded investment securities valued using quoted prices on active exchanges and are therefore classified as Level 1. Deferred Compensation. The deferred compensation plans allow participants to defer certain cash compensation into notional investment accounts. These amounts are included within "Other deferred liabilities and other" in the consolidated balance sheets. The value of certain deferred compensation obligations is based on the market value of the participants' notional investment accounts. The underlying notional investments are comprised primarily of equities, mutual funds, and fixed income securities that are based on directly and indirectly observable market prices. Since the deferred compensation obligations themselves are not exchanged in an active market, they are classified as Level 2. The value of legacy deferred compensation obligations is based on notional investments that earn interest based upon the semiannual rate of U.S. Treasury Bills having a 26-week maturity increased by 1% compounded monthly with a minimum annual rate of 7%, compounded monthly. The notional investments are based upon observable market data, however, since the deferred compensation obligations themselves are not exchanged in an active market, they are classified as Level 2. Derivatives. Derivatives include exchange-traded derivative contracts, over-the-counter transactions, and FTRs. Most exchange-traded derivative contracts are valued based on unadjusted quoted prices in active markets and are therefore classified as Level 1. A small number of exchange-traded derivative contracts are valued using quoted market pricing in markets with insufficient volumes and are therefore considered unobservable and classified as Level 3. Transactions done with an over-the-counter party are on inactive markets and are therefore classified as Level 3. These transactions are valued based on quoted prices from markets with similar exchange-traded transactions. FTRs are priced based upon monthly auction results for identical or similar instruments in a closed market with limited data available and are therefore classified as Level 3.

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- Definition Disclosure of accounting policy for any over or under recovery of the electric fuel-related costs that fall outside a symmetrical cost tolerance band. + References No definition available. + Details
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- Definition Entity's cash and cash equivalents accounting policy with respect to restricted balances. Restrictions may include legally restricted deposits held as compensating balances against short-term borrowing arrangements, contracts entered into with others, or company statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 230 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482913/230-10-50-1 + Details
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- Definition Disclosure of accounting policy for cash and cash equivalents with respect to unrestricted balances. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 230 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482913/230-10-50-1 + Details
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- Definition Disclosure of accounting policy for consolidation to describe the significant judgments and assumptions made in determining whether a variable interest held by the entity requires the variable interest entity to be consolidated and (or) disclose information about its involvement with the variable interest entity; the methodology used by the entity for determining whether or not it is the primary beneficiary of the variable interest entity; and the significant factors considered and judgments made in determining that the power to direct the activities that significantly impact the economic performance of the variable interest entity are shared (as defined). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Paragraph 2AA -Subparagraph (a) -Section 50 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481203/810-10-50-2AA Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (e) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481203/810-10-50-4 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 5A -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481203/810-10-50-5A + Details
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- Definition Disclosure of accounting policy for derivatives used in hedging relationships, which may include how gains or losses are recognized and presented in the financial statements, and amortization policies for deferred amounts. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-1 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 815 -SubTopic 25 -Name Accounting Standards Codification -Section 40 -Paragraph 8A -Publisher FASB -URI https://asc.fasb.org/1943274/2147480270/815-25-40-8A Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4EEEE -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-4EEEE + Details
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- Definition Disclosure of accounting policy for offsetting or not offsetting fair value amounts recognized for derivative instruments against such amounts recognized for the right to reclaim cash collateral (a receivable) or the obligation to return cash collateral (a payable) arising from derivative instruments recognized at fair value. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 8 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-8 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 8 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-8 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 8 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-8 Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-7 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480463/815-10-45-6 + Details
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- Definition Disclosure of accounting policy for computing basic and diluted earnings or loss per share for each class of common stock and participating security. Addresses all significant policy factors, including any antidilutive items that have been excluded from the computation and takes into account stock dividends, splits and reverse splits that occur after the balance sheet date of the latest reporting period but before the issuance of the financial statements. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 260 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482662/260-10-50-1 + Details
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- Definition Disclosure of accounting policy for pension and other postretirement benefit plans. This accounting policy may address (1) the types of plans sponsored by the entity, and the benefits provided by each plan (2) groups that participate in (or are covered by) each plan (3) how plan assets, liabilities and expenses are measured, including the use of any actuaries and (4) significant assumptions used by the entity to value plan assets and liabilities and how such assumptions are derived. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 1 -SubTopic 30 -Topic 715 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481097/715-30-50-1 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 1 -SubTopic 60 -Topic 715 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480266/715-60-50-1 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 1 -SubTopic 70 -Topic 715 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480794/715-70-50-1 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 2 -SubTopic 80 -Topic 715 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480576/715-80-50-2 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 1 -SubTopic 20 -Topic 715 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
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