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Long-Term Debt (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Long-term debt due within one year $ (4,553) $ (24,452)
Unamortized discount and debt issuance costs (4,535) (4,270)
Total Long-Term Debt 493,343 398,161
Long-Term Debt Maturities [Abstract]    
Long term debt maturities in 2019 4,553  
Long term debt maturities in 2020 19,659  
Long term debt maturities in 2021 4,771  
Long term debt maturities in 2022 4,889  
Long term debt maturities in 2023 35,014  
Long term debt maturities in future years 433,545  
First Mortgage Bonds [Member] | 7.70%, 2028 Series    
Debt Instrument [Line Items]    
Long-term debt, gross [1] $ 1,200 $ 1,200
Interest rate 7.70% 7.70%
Debt covenant, allowable amount available for payment of dividends $ 400,000  
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [1] 1,200 $ 1,200
Tax Exempt Debt [Member] | 3.45%, 2027 Series, Industrial Development Revenue Bonds    
Debt Instrument [Line Items]    
Long-term debt, gross $ 19,300 $ 19,300
Interest rate 3.45% 3.45%
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross $ 19,300 $ 19,300
Medium-Term Notes [Member]    
Debt Instrument [Line Items]    
Long-term debt, gross 70,000 70,000
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross 70,000 70,000
Medium-Term Notes [Member] | 6.12%, due 2028    
Debt Instrument [Line Items]    
Long-term debt, gross [2] $ 20,000 $ 20,000
Interest rate 6.12% 6.12%
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [2] $ 20,000 $ 20,000
Medium-Term Notes [Member] | 7.12%, due 2032    
Debt Instrument [Line Items]    
Long-term debt, gross [2] $ 25,000 $ 25,000
Interest rate 7.12% 7.12%
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [2] $ 25,000 $ 25,000
Medium-Term Notes [Member] | 6.247%, due 2037    
Debt Instrument [Line Items]    
Long-term debt, gross [2] $ 25,000 $ 25,000
Interest rate 6.247% 6.247%
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [2] $ 25,000 $ 25,000
Other Long-Term Debt [Member]    
Debt Instrument [Line Items]    
Long-term debt, gross 411,931 336,383
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross 411,931 336,383
Other Long-Term Debt [Member] | 5.59%, due 2018    
Debt Instrument [Line Items]    
Long-term debt, gross [4],[5] 0 [3] $ 20,000
Interest rate   5.59%
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [4],[5] 0 [3] $ 20,000
Other Long-Term Debt [Member] | 3.38%, due 2020    
Debt Instrument [Line Items]    
Long-term debt, gross [4],[5] $ 15,000 $ 15,000
Interest rate 3.38% 3.38%
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [4],[5] $ 15,000 $ 15,000
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 [4],[5] $ 30,000 $ 30,000
Interest rate 3.09% 3.09%
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [4],[5] $ 30,000 $ 30,000
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 [4],[5] $ 15,000 $ 15,000
Interest rate 3.29% 3.29%
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [4],[5] $ 15,000 $ 15,000
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 [4],[5] $ 30,000 $ 30,000
Interest rate 3.11% 3.11%
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [4],[5] $ 30,000 $ 30,000
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] | 5.68%, due 2033    
Debt Instrument [Line Items]    
Long-term debt, gross [4],[6] $ 25,064 $ 26,121
Interest rate 5.68% 5.68%
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [4],[6] $ 25,064 $ 26,121
Other Long-Term Debt [Member] | 5.19%, due 2033    
Debt Instrument [Line Items]    
Long-term debt, gross [4],[6] $ 16,561 $ 17,290
Interest rate 5.19% 5.19%
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [4],[6] $ 16,561 $ 17,290
Other Long-Term Debt [Member] | 5.26%, due 2040    
Debt Instrument [Line Items]    
Long-term debt, gross [4],[5] $ 15,000 $ 15,000
Interest rate 5.26% 5.26%
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [4],[5] $ 15,000 $ 15,000
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] $ 35,139 $ 36,806
Interest rate 5.04% 5.04%
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [4],[7] $ 35,139 $ 36,806
Other Long-Term Debt [Member] | 4.74%, due 2041    
Debt Instrument [Line Items]    
Long-term debt, gross [4],[7] $ 22,167 $ 23,166
Interest rate 4.74% 4.74%
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [4],[7] $ 22,167 $ 23,166
Other Long-Term Debt [Member] | 4.38%, due 2042    
Debt Instrument [Line Items]    
Long-term debt, gross [4],[5] $ 28,000 $ 28,000
Interest rate 4.38% 4.38%
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [4],[5] $ 28,000 $ 28,000
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 [4],[5] $ 20,000 $ 20,000
Interest rate 4.42% 4.42%
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [4],[5] $ 20,000 $ 20,000
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 [4],[5] $ 20,000 $ 20,000
Interest rate 4.47% 4.47%
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [4],[5] $ 20,000 $ 20,000
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 [4],[5] $ 40,000 $ 40,000
Interest rate 3.76% 3.76%
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [4],[5] $ 40,000 $ 40,000
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 [4],[5] $ 60,000 [3] 0
Interest rate 4.19%  
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [4],[5] $ 60,000 [3] 0
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 [4],[5] $ 20,000 [3] 0
Interest rate 4.24%  
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [4],[5] $ 20,000 [3] 0
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 [4],[5] $ 20,000 [3] 0
Interest rate 4.34%  
Long-Term Debt Maturities [Abstract]    
Long-term debt, gross [4],[5] $ 20,000 [3] 0
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 $ (500) (600)
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) $ (100)
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 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, 2018 , approximately $ 400.0 m illion was available for the payment of dividends under this covenant.

[2]

T he indenture under which MGE's Medium-T erm notes are issued provides that those notes will be entitled to be equally and ratably secured in the event that MGE issues any additional firs t mortgage bonds.

[3]

In July 2018, MGE issued a total of $ 4 0 million of new long-term unsecured debt. In September 2018, MGE issued $60 million of new long-term unsecured debt. MGE used the net proceeds from these debt financings to assist with financing capital expenditures, such as the Saratoga Wind Farm, and to refinance $20 million of long-term debt w hich matured in September 2018.

[4]

Unsecured notes issued pursuant to various Note Purchase Agreements with one or more purchasers. The notes are not i ssued under, or governed by, MGE ' s Indenture dated as of September 1, 1998, which governs MGE ' s Medium-Term Notes.

[5]

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 vot ing 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, 2018 , MGE was in compliance with the covenant requirements.

[6]

Issued by MGE Power West Campus. The Note Purchase Agreements require it 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 its ownership interest in the WCCF pursuant to a long-term lease. As of December 31, 2018 , MGE Power West Campus was in compliance with the covenant requirements.

[7]

Issued by M GE 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 its ownership interest in the Elm Road Units pursuant to long-term leases. As of December 31, 2018 , MGE Power Elm Road was in compliance with the covenant requirements.