New Accounting Standards	12 Months Ended
Dec. 31, 2018
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Adoption of Accounting Principles and Recently Issued Accounting Pronouncements	Adoption of Accounting Principles and Recently Issued Accounting Pronouncements - MGE Energy and MGE. a. Recently Adopted Revenue from Contracts with Customers. The FASB issued authoritative guidance within the codification's Revenue Recognition topic that provides guidance on the recognition, measurement, and disclosure of revenue from contracts with customers. The new standard establishes a five-step model for recognizing and measuring revenue from contracts with customers and replaces existing guidance on revenue recognition. The underlying principle is that an entity will recognize revenue to present the transfer of goods or services to customers at an amount that the entity expects to be entitled to in exchange for those goods or services. This authoritative guidance became effective January 1, 2018, and MGE Energy and MGE adopted the standard upon the effective date. Adoption of this standard was permitted under one of two methods: the full retrospective method or the modified retrospective method. MGE Energy and MGE implemented the standard using the modified retrospective method. The cumulative impact of this guidance on our financial statements is not material, except for additional footnote disclosures. See Footnote 19 for further information. Financial Instruments. In January 2016, the FASB issued authoritative guidance within the codification's Financial Instruments topic that provides guidance on the recognition and measurement of financial instruments. This authoritative guidance became effective January 1, 2018, and required equity investments to be measured at fair value with changes in fair value recognized in net income rather than in other comprehensive income. As a result of this guidance, MGE Energy and MGE will no longer have other comprehensive income related to equity investments. This standard was applied using a modified retrospective approach, with a cumulative effect adjustment recorded to opening retained earnings as of the beginning of the fiscal year of adoption. As of January 1, 2018, MGE Energy recorded a $0.4 million increase in retained earnings and a corresponding decrease in accumulated other comprehensive income related to equity investments within the scope of this standard. As of January 1, 2018, MGE recorded less than a $0.1 million decrease in retained earnings and a corresponding increase in accumulated other comprehensive income related to equity investments within the scope of this standard. Restricted Cash. In November 2016, the FASB issued authoritative guidance within the codification's Statement of Cash Flows topic that provides guidance on the classification and presentation of changes in restricted cash within the statement of cash flows. Under the new guidance, reporting entities are required to explain the changes in the total of restricted and unrestricted cash and cash equivalents when reconciling the beginning and ending balances on the statement of cash flows. Prior to the authoritative guidance, changes in restricted cash were presented as either cash flows from operating, investing, or financing activities within the statement of cash flows based on the nature of the restriction. Reporting entities are now also required to provide a reconciliation from the balance sheet to the statement of cash flows and disclose the nature of the restrictions of cash. This authoritative guidance became effective January 1, 2018. Upon the effective date, MGE Energy and MGE changed the presentation of restricted cash on the consolidated statements of cash flows to reflect the new accounting guidance retrospectively for all periods presented. See Footnote 1.d. for further information. Pension and Other Postretirement Benefits. In March 2017, the FASB issued authoritative guidance within the codification's Compensation – Retirement Benefits topic that provides guidance on the presentation of net periodic pension cost and net periodic postretirement benefit cost (together, net benefit cost). This authoritative guidance became effective January 1, 2018. Under the new guidance, the service cost component of net benefit cost is required to be recorded in the same line item or items as other compensation costs arising from services rendered by the pertinent employees during the period. The other components of net benefit cost are required to be presented in the income statement separately from the service cost component and outside of income from operations. A practical expedient within the standard permits an employer to use the amounts disclosed in its pension and other postretirement benefit plan footnote for prior comparative periods as the estimation basis for applying the retrospective presentation requirements. MGE Energy and MGE have elected to apply the practical expedient. Upon the effective date, MGE Energy and MGE changed the presentation of net benefit cost on the consolidated statements of income to reflect the new accounting guidance retrospectively to all periods presented. For both MGE Energy and MGE, "Other operations and maintenance expense" increased and "Other income, net" increased $4.1 million and $4.3 million for the years ended December 31, 2017 and 2016, respectively. The standard also only allows the service cost component to be eligible for capitalization prospectively from the effective date of the pronouncement (whereas under previous GAAP all components of net benefit cost were eligible for capitalization). See Footnote 13 for further information. Internal-Use Software – Hosting Arrangements. In August 2018, the FASB issued amended authoritative guidance within the codification's Intangibles – Goodwill and Other – Internal-Use Software topic. The amended authoritative guidance aligns the requirements for capitalizing implementation costs incurred in a cloud computing arrangement (hosting arrangement) that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangement that include an internal-use software license). Costs for implementation activities in the development stage are capitalized depending on the nature of the costs and presented in the same line item on the balance sheet as amounts prepaid for the hosted service. Costs incurred during the preliminary and postimplementation stages are expensed as the activities are performed. The costs capitalized as part of implementation stage should be expensed over the term of the hosting contract, which includes any renewable option periods, and presented in the same line on the income statement as the fees for the associated hosted service. This amended authoritative guidance will become effective January 1, 2020. Early adoption of the amendment is permitted, including adoption in any interim period. Entities can choose to adopt the new guidance either prospectively, for eligible costs incurred on or after the date this guidance is first applied, or retrospectively. MGE Energy and MGE early adopted these amendments retrospectively as of September 30, 2018. The cumulative impact of this guidance on our financial statement was not material, except for additional footnote disclosures. See Footnote 1.t. for disclosures required under this standard. b. Recently Issued Leases. In February 2016, the FASB issued authoritative guidance within the codification's Leases topic that provides guidance on the classification, recognition, measurement, and disclosure of leases. The new leasing standard establishes that a lease conveys the right to control the use of identified property, plant, or equipment for a period of time in exchange for consideration. Under the new guidance, lessees will be required to recognize all leases with terms greater than one year, including operating leases, on the balance sheet by recording a right-of-use asset and lease liability. Prior to the authoritative guidance, only capital leases were recognized on the balance sheet by lessees. The new accounting guidance, as applied by lessors, is materially consistent with current GAAP. In January 2018, the FASB issued authoritative guidance which provided an optional practical expedient to grandfather the accounting for existing and expired land easements not accounted for as a lease under the new authoritative guidance. MGE Energy and MGE adopted this practical expedient. Management has completed a bottoms-up approach to analyze the impact of the standard on our lease portfolio. MGE Energy and MGE have reviewed current accounting policies and procedures to identify potential differences in accounting treatment that would result from applying the requirements of the new standard to our existing lease portfolio. In addition, we identified appropriate changes to our business processes, systems, and controls to support recognition and disclosure requirements under the new standard. This authoritative guidance became effective January 1, 2019. MGE Energy and MGE adopted the standard upon the effective date. In compliance with authorized transition guidance, MGE Energy and MGE began applying the new standard on January 1, 2019, but will continue to present periods prior to that date according to the previous authoritative standard. MGE Energy and MGE expect to recognize approximately $15-$17 million additional lease assets and liabilities under the new standard. We do not expect that it will have a material impact on our consolidated net income or cash flows.

Adoption of Accounting Principles and Recently Issued Accounting Pronouncements - MGE Energy and MGE.

a. Recently Adopted

Revenue from Contracts with Customers.

The FASB issued authoritative guidance within the codification's Revenue Recognition topic that provides guidance on the recognition, measurement, and disclosure of revenue from contracts with customers. The new standard establishes a five-step model for recognizing and measuring revenue from contracts with customers and replaces existing guidance on revenue recognition. The underlying principle is that an entity will recognize revenue to present the transfer of goods or services to customers at an amount that the entity expects to be entitled to in exchange for those goods or services.

This authoritative guidance became effective January 1, 2018, and MGE Energy and MGE adopted the standard upon the effective date. Adoption of this standard was permitted under one of two methods: the full retrospective method or the modified retrospective method. MGE Energy and MGE implemented the standard using the modified retrospective method. The cumulative impact of this guidance on our financial statements is not material, except for additional footnote disclosures. See Footnote 19 for further information.

Financial Instruments.

In January 2016, the FASB issued authoritative guidance within the codification's Financial Instruments topic that provides guidance on the recognition and measurement of financial instruments. This authoritative guidance became effective January 1, 2018, and required equity investments to be measured at fair value with changes in fair value recognized in net income rather than in other comprehensive income. As a result of this guidance, MGE Energy and MGE will no longer have other comprehensive income related to equity investments. This standard was applied using a modified retrospective approach, with a cumulative effect adjustment recorded to opening retained earnings as of the beginning of the fiscal year of adoption. As of January 1, 2018, MGE Energy recorded a $0.4 million increase in retained earnings and a corresponding decrease in accumulated other comprehensive income related to equity investments within the scope of this standard. As of January 1, 2018, MGE recorded less than a $0.1 million decrease in retained earnings and a corresponding increase in accumulated other comprehensive income related to equity investments within the scope of this standard.

Restricted Cash.

In November 2016, the FASB issued authoritative guidance within the codification's Statement of Cash Flows topic that provides guidance on the classification and presentation of changes in restricted cash within the statement of cash flows. Under the new guidance, reporting entities are required to explain the changes in the total of restricted and unrestricted cash and cash equivalents when reconciling the beginning and ending balances on the statement of cash flows. Prior to the authoritative guidance, changes in restricted cash were presented as either cash flows from operating, investing, or financing activities within the statement of cash flows based on the nature of the restriction. Reporting entities are now also required to provide a reconciliation from the balance sheet to the statement of cash flows and disclose the nature of the restrictions of cash. This authoritative guidance became effective January 1, 2018. Upon the effective date, MGE Energy and MGE changed the presentation of restricted cash on the consolidated statements of cash flows to reflect the new accounting guidance retrospectively for all periods presented. See Footnote 1.d. for further information.

Pension and Other Postretirement Benefits.

In March 2017, the FASB issued authoritative guidance within the codification's Compensation – Retirement Benefits topic that provides guidance on the presentation of net periodic pension cost and net periodic postretirement benefit cost (together, net benefit cost). This authoritative guidance became effective January 1, 2018. Under the new guidance, the service cost component of net benefit cost is required to be recorded in the same line item or items as other compensation costs arising from services rendered by the pertinent employees during the period. The other components of net benefit cost are required to be presented in the income statement separately from the service cost component and outside of income from operations. A practical expedient within the standard permits an employer to use the amounts disclosed in its pension and other postretirement benefit plan footnote for prior comparative periods as the estimation basis for applying the retrospective presentation requirements. MGE Energy and MGE have elected to apply the practical expedient. Upon the effective date, MGE Energy and MGE changed the presentation of net benefit cost on the consolidated statements of income to reflect the new accounting guidance retrospectively to all periods presented. For both MGE Energy and MGE, "Other operations and maintenance expense" increased and "Other income, net" increased $4.1 million and $4.3 million for the years ended December 31, 2017 and 2016, respectively. The standard also only allows the service cost component to be eligible for capitalization prospectively from the effective date of the pronouncement (whereas under previous GAAP all components of net benefit cost were eligible for capitalization). See Footnote 13 for further information.

Internal-Use Software – Hosting Arrangements.

In August 2018, the FASB issued amended authoritative guidance within the codification's Intangibles – Goodwill and Other – Internal-Use Software topic. The amended authoritative guidance aligns the requirements for capitalizing implementation costs incurred in a cloud computing arrangement (hosting arrangement) that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangement that include an internal-use software license). Costs for implementation activities in the development stage are capitalized depending on the nature of the costs and presented in the same line item on the balance sheet as amounts prepaid for the hosted service. Costs incurred during the preliminary and postimplementation stages are expensed as the activities are performed. The costs capitalized as part of implementation stage should be expensed over the term of the hosting contract, which includes any renewable option periods, and presented in the same line on the income statement as the fees for the associated hosted service.

This amended authoritative guidance will become effective January 1, 2020. Early adoption of the amendment is permitted, including adoption in any interim period. Entities can choose to adopt the new guidance either prospectively, for eligible costs incurred on or after the date this guidance is first applied, or retrospectively. MGE Energy and MGE early adopted these amendments retrospectively as of September 30, 2018. The cumulative impact of this guidance on our financial statement was not material, except for additional footnote disclosures. See Footnote 1.t. for disclosures required under this standard.

b. Recently Issued

Leases.

In February 2016, the FASB issued authoritative guidance within the codification's Leases topic that provides guidance on the classification, recognition, measurement, and disclosure of leases. The new leasing standard establishes that a lease conveys the right to control the use of identified property, plant, or equipment for a period of time in exchange for consideration. Under the new guidance, lessees will be required to recognize all leases with terms greater than one year, including operating leases, on the balance sheet by recording a right-of-use asset and lease liability. Prior to the authoritative guidance, only capital leases were recognized on the balance sheet by lessees. The new accounting guidance, as applied by lessors, is materially consistent with current GAAP. In January 2018, the FASB issued authoritative guidance which provided an optional practical expedient to grandfather the accounting for existing and expired land easements not accounted for as a lease under the new authoritative guidance. MGE Energy and MGE adopted this practical expedient.

Management has completed a bottoms-up approach to analyze the impact of the standard on our lease portfolio. MGE Energy and MGE have reviewed current accounting policies and procedures to identify potential differences in accounting treatment that would result from applying the requirements of the new standard to our existing lease portfolio. In addition, we identified appropriate changes to our business processes, systems, and controls to support recognition and disclosure requirements under the new standard. This authoritative guidance became effective January 1, 2019. MGE Energy and MGE adopted the standard upon the effective date. In compliance with authorized transition guidance, MGE Energy and MGE began applying the new standard on January 1, 2019, but will continue to present periods prior to that date according to the previous authoritative standard. MGE Energy and MGE expect to recognize approximately $15-$17 million additional lease assets and liabilities under the new standard. We do not expect that it will have a material impact on our consolidated net income or cash flows.