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Long-Term Debt (Details-1) - USD ($) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2019 | Dec. 31, 2018
Debt Instrument [Line Items] |  |  |
Long-term debt due within one year |  | $ (19,659,000) | $ (4,553,000)
Unamortized discount and debt issuance costs |  | (4,479,000) | (4,535,000)
Total Long-Term Debt |  | 523,741,000 | 493,343,000
First Mortgage Bonds [Member] | 7.70%, 2028 Series |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [1] | $ 1,200,000 | $ 1,200,000
Interest rate |  | 7.70% | 7.70%
Debt covenant, allowable amount available for payment of dividends |  | $ 465,200,000 |
Tax Exempt Debt [Member] | 3.45%, 2027 Series, Industrial Development Revenue Bonds |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross |  | $ 19,300,000 | $ 19,300,000
Interest rate |  | 3.45% | 3.45%
Medium-Term Notes [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross |  | $ 70,000,000 | $ 70,000,000
Medium-Term Notes [Member] | 6.12%, due 2028 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [2] | $ 20,000,000 | $ 20,000,000
Interest rate |  | 6.12% | 6.12%
Medium-Term Notes [Member] | 7.12%, due 2032 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [2] | $ 25,000,000 | $ 25,000,000
Interest rate |  | 7.12% | 7.12%
Medium-Term Notes [Member] | 6.247%, due 2037 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [2] | $ 25,000,000 | $ 25,000,000
Interest rate |  | 6.247% | 6.247%
Other Long-Term Debt [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross |  | $ 457,379,000 | $ 411,931,000
Other Long-Term Debt [Member] | 3.38%, due 2020 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [3],[4] | $ 15,000,000 | $ 15,000,000
Interest rate |  | 3.38% | 3.38%
Other Long-Term Debt [Member] | 3.38%, due 2020 | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt covenant, ownership interest by outside party resulting in repurchase of notes at par value |  | 30.00% |
Other Long-Term Debt [Member] | 3.38%, due 2020 | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt to total capitalization ratio |  | 0.65 |
Priority debt to assets ratio |  | 0.2 |
Other Long-Term Debt [Member] | 3.09%, due 2023 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [3],[4] | $ 30,000,000 | $ 30,000,000
Interest rate |  | 3.09% | 3.09%
Other Long-Term Debt [Member] | 3.09%, due 2023 | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt covenant, ownership interest by outside party resulting in repurchase of notes at par value |  | 30.00% |
Other Long-Term Debt [Member] | 3.09%, due 2023 | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt to total capitalization ratio |  | 0.65 |
Priority debt to assets ratio |  | 0.2 |
Other Long-Term Debt [Member] | 3.29%, due 2026 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [3],[4] | $ 15,000,000 | $ 15,000,000
Interest rate |  | 3.29% | 3.29%
Other Long-Term Debt [Member] | 3.29%, due 2026 | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt covenant, ownership interest by outside party resulting in repurchase of notes at par value |  | 30.00% |
Other Long-Term Debt [Member] | 3.29%, due 2026 | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt to total capitalization ratio |  | 0.65 |
Priority debt to assets ratio |  | 0.2 |
Other Long-Term Debt [Member] | 3.11% due 2027 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [3],[4] | $ 30,000,000 | $ 30,000,000
Interest rate |  | 3.11% | 3.11%
Other Long-Term Debt [Member] | 3.11% due 2027 | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt covenant, ownership interest by outside party resulting in repurchase of notes at par value |  | 30.00% |
Other Long-Term Debt [Member] | 3.11% due 2027 | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt to total capitalization ratio |  | 0.65 |
Priority debt to assets ratio |  | 0.2 |
Other Long-Term Debt [Member] | 2.94% due 2029 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [3],[4],[5] | $ 50,000,000 | $ 0
Interest rate |  | 2.94% |
Other Long-Term Debt [Member] | 2.94% due 2029 | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt covenant, ownership interest by outside party resulting in repurchase of notes at par value |  | 30.00% |
Other Long-Term Debt [Member] | 2.94% due 2029 | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt to total capitalization ratio |  | 0.65 |
Priority debt to assets ratio |  | 0.2 |
Other Long-Term Debt [Member] | 5.68%, due 2033 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [4],[6] | $ 23,946,000 | $ 25,064,000
Interest rate |  | 5.68% | 5.68%
Other Long-Term Debt [Member] | 5.19%, due 2033 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [4],[6] | $ 15,794,000 | $ 16,561,000
Interest rate |  | 5.19% | 5.19%
Other Long-Term Debt [Member] | 5.26%, due 2040 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [3],[4] | $ 15,000,000 | $ 15,000,000
Interest rate |  | 5.26% | 5.26%
Other Long-Term Debt [Member] | 5.26%, due 2040 | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt covenant, ownership interest by outside party resulting in repurchase of notes at par value |  | 30.00% |
Other Long-Term Debt [Member] | 5.26%, due 2040 | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt to total capitalization ratio |  | 0.65 |
Priority debt to assets ratio |  | 0.2 |
Other Long-Term Debt [Member] | 5.04%, due 2040 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [4],[7] | $ 33,472,000 | $ 35,139,000
Interest rate |  | 5.04% | 5.04%
Other Long-Term Debt [Member] | 4.74%, due 2041 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [4],[7] | $ 21,167,000 | $ 22,167,000
Interest rate |  | 4.74% | 4.74%
Other Long-Term Debt [Member] | 4.38%, due 2042 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [3],[4] | $ 28,000,000 | $ 28,000,000
Interest rate |  | 4.38% | 4.38%
Other Long-Term Debt [Member] | 4.38%, due 2042 | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt covenant, ownership interest by outside party resulting in repurchase of notes at par value |  | 30.00% |
Other Long-Term Debt [Member] | 4.38%, due 2042 | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt to total capitalization ratio |  | 0.65 |
Priority debt to assets ratio |  | 0.2 |
Other Long-Term Debt [Member] | 4.42%, due 2043 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [3],[4] | $ 20,000,000 | $ 20,000,000
Interest rate |  | 4.42% | 4.42%
Other Long-Term Debt [Member] | 4.42%, due 2043 | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt covenant, ownership interest by outside party resulting in repurchase of notes at par value |  | 30.00% |
Other Long-Term Debt [Member] | 4.42%, due 2043 | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt to total capitalization ratio |  | 0.65 |
Priority debt to assets ratio |  | 0.2 |
Other Long-Term Debt [Member] | 4.47%, due 2048 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [3],[4] | $ 20,000,000 | $ 20,000,000
Interest rate |  | 4.47% | 4.47%
Other Long-Term Debt [Member] | 4.47%, due 2048 | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt covenant, ownership interest by outside party resulting in repurchase of notes at par value |  | 30.00% |
Other Long-Term Debt [Member] | 4.47%, due 2048 | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt to total capitalization ratio |  | 0.65 |
Priority debt to assets ratio |  | 0.2 |
Other Long-Term Debt [Member] | 3.76% due 2052 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [3],[4] | $ 40,000,000 | $ 40,000,000
Interest rate |  | 3.76% | 3.76%
Other Long-Term Debt [Member] | 3.76% due 2052 | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt covenant, ownership interest by outside party resulting in repurchase of notes at par value |  | 30.00% |
Other Long-Term Debt [Member] | 3.76% due 2052 | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt to total capitalization ratio |  | 0.65 |
Priority debt to assets ratio |  | 0.2 |
Other Long-Term Debt [Member] | 4.19% due 2048 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [3],[4] | $ 60,000,000 | $ 60,000,000
Interest rate |  | 4.19% | 4.19%
Other Long-Term Debt [Member] | 4.19% due 2048 | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt covenant, ownership interest by outside party resulting in repurchase of notes at par value |  | 30.00% |
Other Long-Term Debt [Member] | 4.19% due 2048 | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt to total capitalization ratio |  | 0.65 |
Priority debt to assets ratio |  | 0.2 |
Other Long-Term Debt [Member] | 4.24% due 2053 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [3],[4] | $ 20,000,000 | $ 20,000,000
Interest rate |  | 4.24% | 4.24%
Other Long-Term Debt [Member] | 4.24% due 2053 | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt covenant, ownership interest by outside party resulting in repurchase of notes at par value |  | 30.00% |
Other Long-Term Debt [Member] | 4.24% due 2053 | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt to total capitalization ratio |  | 0.65 |
Priority debt to assets ratio |  | 0.2 |
Other Long-Term Debt [Member] | 4.34% due 2058 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt, gross | [3],[4] | $ 20,000,000 | $ 20,000,000
Interest rate |  | 4.34% | 4.34%
Other Long-Term Debt [Member] | 4.34% due 2058 | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt covenant, ownership interest by outside party resulting in repurchase of notes at par value |  | 30.00% |
Other Long-Term Debt [Member] | 4.34% due 2058 | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt to total capitalization ratio |  | 0.65 |
Priority debt to assets ratio |  | 0.2 |
MGE Power Elm Road [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Unamortized discount and debt issuance costs |  | $ (400,000) | $ (500,000)
MGE Power Elm Road [Member] | Other Long-Term Debt [Member] | 5.04%, due 2040 | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt service coverage ratio |  | 1.25 |
MGE Power Elm Road [Member] | Other Long-Term Debt [Member] | 4.74%, due 2041 | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt service coverage ratio |  | 1.25 |
MGE Power West Campus [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Unamortized discount and debt issuance costs |  | $ (100,000) | $ (100,000)
MGE Power West Campus [Member] | Other Long-Term Debt [Member] | 5.68%, due 2033 | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt service coverage ratio |  | 1.25 |
MGE Power West Campus [Member] | Other Long-Term Debt [Member] | 5.68%, due 2033 | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt to total capitalization ratio |  | 0.65 |
MGE Power West Campus [Member] | Other Long-Term Debt [Member] | 5.19%, due 2033 | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt service coverage ratio |  | 1.25 |
MGE Power West Campus [Member] | Other Long-Term Debt [Member] | 5.19%, due 2033 | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt to total capitalization ratio |  | 0.65 |
[1] | MGE's utility plant is subject to the lien of its Indenture of Mortgage and Deed of Trust, under which its first mortgage bonds are issued. The Mortgage Indenture provides that dividends or any other distribution or purchase of MGE shares may not be made if the aggregate amount thereof since December 31, 1945, would exceed the earned surplus (retained earnings) accumulated subsequent to December 31, 1945. As of December 31, 2019, approximately $465.2 million was available for the payment of dividends under this covenant.
[2] | The indenture under which MGE's Medium-Term notes are issued provides that those notes will be entitled to be equally and ratably secured in the event that MGE issues any additional first mortgage bonds.
[3] | Issued by MGE. Under that Note Purchase Agreement: (i) note holders have the right to require MGE to repurchase their notes at par in the event of an acquisition of beneficial ownership of 30% or more of the outstanding voting stock of MGE Energy, (ii) MGE must maintain a ratio of its consolidated indebtedness to consolidated total capitalization not to exceed a maximum of 65%, and (iii) MGE cannot issue "Priority Debt" in an amount exceeding 20% of its consolidated assets. Priority Debt is defined as any indebtedness of MGE secured by liens other than specified liens permitted by the Note Purchase Agreement and certain unsecured indebtedness of certain subsidiaries. As of December 31, 2019, MGE was in compliance with the covenant requirements.
[4] | Unsecured notes issued pursuant to various Note Purchase Agreements with one or more purchasers. The notes are not issued under, or governed by, MGE's Indenture dated as of September 1, 1998, which governs MGE's Medium-Term Notes.
[5] | In November 2019, MGE issued $50 million of new long-term unsecured debt. The proceeds of the debt financing were used to assist with capital expenditures, maturing short-term debt, and other corporate obligations.
[6] | Issued by MGE Power West Campus. The Note Purchase Agreements require MGE Power West Campus to maintain a projected debt service coverage ratio of not less than 1.25 to 1.00, and debt to total capitalization ratio of not more than 0.65 to 1.00. The notes are secured by a collateral assignment of lease payments that MGE is making to MGE Power West Campus for use of the WCCF pursuant to a long-term lease. As of December 31, 2019, MGE Power West Campus was in compliance with the covenant requirements.
[7] | Issued by MGE Power Elm Road. The Note Purchase Agreement requires MGE Power Elm Road to maintain a projected and actual debt service coverage ratio at the end of any calendar quarter of not less than 1.25 to 1.00 for the trailing 12-month period. The notes are secured by a collateral assignment of lease payments that MGE is making to MGE Power Elm Road for use of the Elm Road Units pursuant to long-term leases. As of December 31, 2019, MGE Power Elm Road was in compliance with the covenant requirements.

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