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Property, Plant, and Equipment (Details) - USD ($) $ in Thousands | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | ---
Property, Plant and Equipment [Line Items] |  |  |
In-service utility plant, net |  | $ 1,217,094 | $ 1,189,077
Construction work in progress |  | 26,351 | 19,029
Total property, plant, and equipment |  | 1,243,445 | 1,208,106
First Mortgage Bonds [Member] | 7.70%, 2028 Series |  |  |
Plant related debt items |  |  |
Long-term debt, gross | [1] | 1,200 | 1,200
Regulated [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
In-service utility plant, gross |  | 1,531,864 | 1,480,928
Less: Accumulated depreciation and amortization |  | 578,410 | 559,615
In-service utility plant, net |  | 953,454 | 921,313
Construction work in progress |  | 23,837 | 16,988
Electric [Member] | Regulated [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
In-service utility plant, gross |  | 1,147,701 | 1,110,953
Gas [Member] | Regulated [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
In-service utility plant, gross |  | 384,163 | 369,975
Non Regulated Energy [Member] | Non-regulated [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
In-service utility plant, gross |  | 315,589 | 313,152
Less: Accumulated depreciation and amortization |  | 51,949 | 45,388
In-service utility plant, net |  | 263,640 | 267,764
Construction work in progress |  | 2,514 | 2,041
MGE [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
In-service utility plant, net |  | 1,216,415 | 1,188,351
Construction work in progress |  | 26,351 | 19,029
Total property, plant, and equipment |  | 1,242,766 | 1,207,380
MGE [Member] | First Mortgage Bonds [Member] | 7.70%, 2028 Series |  |  |
Plant related debt items |  |  |
Long-term debt, gross | [1] | 1,200 | 1,200
MGE [Member] | Regulated [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
In-service utility plant, gross |  | 1,531,893 | 1,480,957
Less: Accumulated depreciation and amortization |  | 578,410 | 559,615
In-service utility plant, net |  | 953,483 | 921,342
Construction work in progress |  | 23,837 | 16,988
MGE [Member] | Electric [Member] | Regulated [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
In-service utility plant, gross |  | 1,147,718 | 1,110,970
MGE [Member] | Gas [Member] | Regulated [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
In-service utility plant, gross |  | 384,175 | 369,987
MGE [Member] | Non Regulated Energy [Member] | Non-regulated [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
In-service utility plant, gross |  | 314,750 | 312,314
Less: Accumulated depreciation and amortization |  | 51,818 | 45,305
In-service utility plant, net |  | 262,932 | 267,009
Construction work in progress |  | $ 2,514 | $ 2,041
[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, 2015 , approximately $ 353.0 million was available for the payment of dividends under this covenant.

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- Definition Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. This amount does not include Construction work in progress. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures. + References No definition available. + Details
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