Property, Plant, and Equipment (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
In-service utility plant, net		$ 1,283,313	$ 1,245,269
Construction work in progress		58,044	36,790
Total property, plant, and equipment		1,341,357	1,282,059
Assets held for sale		8,817	14,813
Plant related debt items
Long-term debt, gross		426,883
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		1,624,878	1,566,794
Less: Accumulated depreciation and amortization	[2]	599,469	579,965
In-service utility plant, net		1,025,409	986,829
Construction work in progress	[2]	56,208	31,356
Electric [Member] | Regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross	[2]	1,209,336	1,171,004
Gas [Member] | Regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross		415,542	395,790
Non Regulated Energy [Member] | Non-regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross		319,611	317,810
Less: Accumulated depreciation and amortization		61,707	59,370
In-service utility plant, net		257,904	258,440
Construction work in progress		1,836	5,434
MGE [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, net		1,283,341	1,244,648
Construction work in progress		58,044	36,790
Total property, plant, and equipment		1,341,385	1,281,438
Assets held for sale		8,817	14,813
Plant related debt items
Long-term debt, gross		426,883
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		1,624,906	1,566,822
Less: Accumulated depreciation and amortization	[2]	599,469	579,965
In-service utility plant, net		1,025,437	986,857
Construction work in progress	[2]	56,208	31,356
MGE [Member] | Electric [Member] | Regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross	[2]	1,209,353	1,171,021
MGE [Member] | Gas [Member] | Regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross		415,553	395,801
MGE [Member] | Non Regulated Energy [Member] | Non-regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross		319,611	317,014
Less: Accumulated depreciation and amortization		61,707	59,223
In-service utility plant, net		257,904	257,791
Construction work in progress		$ 1,836	$ 5,434

[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, 2017 , approximately $ 343.3 m illion was available for the payment of dividends under this covenant.

[2] As of December 31, 2017 and 2016 , MGE has classified $ 8.8 million and $ 14.8 million , respectively, of Columbia assets as held-for-sale on the consolidated balance sheets related to the partial sale of plant assets to WPL . See Footnote 5.a. for further discussion.