Adoption of Accounting Principles and Recently Issued Accounting Pronouncements	9 Months Ended
Sep. 30, 2017
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
New Accounting Pronouncements	Adoption of Accounting Principles and Recently Issued Accounting Pronouncements - MGE Energy and MGE. a. Revenue from Contracts with Customers. In May 2014, 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 objective of the new standard is to provide a single, comprehensive revenue recognition model for all contracts with customers to improve comparability within industries, across industries and across capital markets. The underlying principle is that an entity will recognize revenue to depict 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. MGE Energy and MGE have been assessing the impact of this guidance on revenue streams within the scope of the new standard. All retail electric and gas revenues are tariff rates approved by the PSCW. Based on our evaluation of the new standard, retail revenues will be recognized within the period in which utility service is provided to the customer and the performance obligation is fulfilled, consistent with our current revenue recognition model. Electric revenues for sales to the market represent wholesale sales made to third parties who are not ultimate users of the electricity. These sales may also include bilateral sales to other utilities or power marketers. Revenues for sales to the market will be recognized when the sale is completed within the market operated by MISO, similar to the recognition under our current revenue recognition model. In addition, revenues from the transportation of gas will continue to be recognized upon the performance of services for the respective customer. Based on our assessment of the new standard, revenue recognition for retail revenues, sales to the market, and transportation of gas will be materially consistent with our current revenue recognition model. However, additional disclosures regarding the nature, amount, timing, and uncertainty of these revenue streams and related cash flows arising from contracts with customers will be required as a result of the new standard. Management continues to analyze newly-released interpretative guidance and assess the related impacts to the current revenue recognition model. This authoritative guidance will become effective January 1, 2018, and MGE Energy and MGE anticipate adopting the standard upon the effective date. Adoption of this standard is permitted under one of two methods: the full retrospective method or the modified retrospective method. MGE Energy and MGE are continuing to assess the permitted implementation methods and the impact on our financial statements. b. 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 will become effective January 1, 2018, and will require 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 any other comprehensive income related to equity investments. This standard will be applied using a modified retrospective approach, with a cumulative effect adjustment recorded to opening retained earnings as of the beginning of all prior periods presented. As of September 30, 2017, MGE Energy had $0.4 million and MGE had less than $0.1 million of accumulated other comprehensive income related to equity investments within the scope of this standard. c. 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 consolidated 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 from that applied under current GAAP. Management has begun utilizing a bottoms-up approach to analyze the impact of the standard on our lease portfolio. MGE Energy and MGE have been reviewing 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 are identifying appropriate changes to our business processes, systems, and controls to support recognition and disclosure requirements under the new standard. This authoritative guidance will become effective January 1, 2019, with early adoption permitted. MGE Energy and MGE anticipate adopting the standard upon the effective date. The new leasing standard requires entities to recognize and measure leases at the beginning of the earliest comparative period presented using a modified retrospective approach. MGE Energy and MGE are currently assessing the impact this pronouncement will have on our financial statements. d. 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. The new standard was issued to eliminate a current diversity in practice for the accounting treatment of restricted cash. Under the new guidance, reporting entities will be 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, as appropriate based on the nature of the restriction. Also under the new standard, reporting entities will be 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 will become effective January 1, 2018. Upon the effective date, MGE Energy and MGE will change the presentation of restricted cash to reflect this change in accounting guidance. MGE Energy and MGE will also retrospectively apply the guidance to all prior periods presented. As of September 30, 2017, and December 31, 2016, MGE Energy and MGE had $4.5 million and $5.1 million, respectively, of restricted cash classified within other current assets on the consolidated balance sheets. e. Pension and Other Postretirement Benefits. In March 2017, the FASB issued authoritative guidance within the 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 will become 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. Operating income will decrease and other income will increase $6.6 million and $5.0 million for the years ended December 31, 2016 and 2015, 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 current GAAP, all components of net benefit cost are eligible for capitalization). MGE Energy and MGE are currently evaluating the impact of how the change in components of net benefit cost eligible for capitalization will affect our financial statements.

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

a. Revenue from Contracts with Customers.

In May 2014, 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 objective of the new standard is to provide a single, comprehensive revenue recognition model for all contracts with customers to improve comparability within industries, across industries and across capital markets. The underlying principle is that an entity will recognize revenue to depict 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.

MGE Energy and MGE have been assessing the impact of this guidance on revenue streams within the scope of the new standard. All retail electric and gas revenues are tariff rates approved by the PSCW. Based on our evaluation of the new standard, retail revenues will be recognized within the period in which utility service is provided to the customer and the performance obligation is fulfilled, consistent with our current revenue recognition model. Electric revenues for sales to the market represent wholesale sales made to third parties who are not ultimate users of the electricity. These sales may also include bilateral sales to other utilities or power marketers. Revenues for sales to the market will be recognized when the sale is completed within the market operated by MISO, similar to the recognition under our current revenue recognition model. In addition, revenues from the transportation of gas will continue to be recognized upon the performance of services for the respective customer. Based on our assessment of the new standard, revenue recognition for retail revenues, sales to the market, and transportation of gas will be materially consistent with our current revenue recognition model. However, additional disclosures regarding the nature, amount, timing, and uncertainty of these revenue streams and related cash flows arising from contracts with customers will be required as a result of the new standard. Management continues to analyze newly-released interpretative guidance and assess the related impacts to the current revenue recognition model.

This authoritative guidance will become effective January 1, 2018, and MGE Energy and MGE anticipate adopting the standard upon the effective date. Adoption of this standard is permitted under one of two methods: the full retrospective method or the modified retrospective method. MGE Energy and MGE are continuing to assess the permitted implementation methods and the impact on our financial statements.

b. 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 will become effective January 1, 2018, and will require 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 any other comprehensive income related to equity investments. This standard will be applied using a modified retrospective approach, with a cumulative effect adjustment recorded to opening retained earnings as of the beginning of all prior periods presented. As of September 30, 2017, MGE Energy had $0.4 million and MGE had less than $0.1 million of accumulated other comprehensive income related to equity investments within the scope of this standard.

c. 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 consolidated 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 from that applied under current GAAP.

Management has begun utilizing a bottoms-up approach to analyze the impact of the standard on our lease portfolio. MGE Energy and MGE have been reviewing 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 are identifying appropriate changes to our business processes, systems, and controls to support recognition and disclosure requirements under the new standard. This authoritative guidance will become effective January 1, 2019, with early adoption permitted. MGE Energy and MGE anticipate adopting the standard upon the effective date. The new leasing standard requires entities to recognize and measure leases at the beginning of the earliest comparative period presented using a modified retrospective approach. MGE Energy and MGE are currently assessing the impact this pronouncement will have on our financial statements.

d. 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. The new standard was issued to eliminate a current diversity in practice for the accounting treatment of restricted cash. Under the new guidance, reporting entities will be 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, as appropriate based on the nature of the restriction. Also under the new standard, reporting entities will be 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 will become effective January 1, 2018. Upon the effective date, MGE Energy and MGE will change the presentation of restricted cash to reflect this change in accounting guidance. MGE Energy and MGE will also retrospectively apply the guidance to all prior periods presented. As of September 30, 2017, and December 31, 2016, MGE Energy and MGE had $4.5 million and $5.1 million, respectively, of restricted cash classified within other current assets on the consolidated balance sheets.

e. Pension and Other Postretirement Benefits.

In March 2017, the FASB issued authoritative guidance within the 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 will become 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. Operating income will decrease and other income will increase $6.6 million and $5.0 million for the years ended December 31, 2016 and 2015, 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 current GAAP, all components of net benefit cost are eligible for capitalization). MGE Energy and MGE are currently evaluating the impact of how the change in components of net benefit cost eligible for capitalization will affect our financial statements.