Property, Plant, and Equipment (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2018 USD ($) Turbines MW	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Property, Plant and Equipment [Line Items]
In-service utility plant, net		$ 1,369,766	$ 1,283,313
Construction work in progress		139,671	58,044
Total property, plant, and equipment		1,509,437	1,341,357
Assets held for sale		$ 3,080	8,817
Forward Wind [Abstract]
Forward Wind Number Of Operating Turbines | Turbines		86
Forward Wind Proportionate Purchase Price		$ 23,000
Forward Wind Aggregate Purchase Price		$ 174,000
Jointly Owned Utility Plant Proportionate Ownership Share		12.80%
Saratoga Wind [Abstract]
Saratoga Megawatts | MW		66
Saratoga Turbines | Turbines		33
Saratoga Wind Project Costs		$ 95,800
Allowance for Funds Used During Construction
Authorized AFUDC rate		100.00%
AFUDC equity recognized		$ 2,500
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		1,753,002	1,624,878
Less: Accumulated depreciation and amortization	[2],[3]	639,486	599,469
In-service utility plant, net		1,113,516	1,025,409
Construction work in progress	[2],[4]	138,663	56,208
Electric [Member] | Regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross	[2],[3]	1,310,421	1,209,336
Gas [Member] | Regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross		442,581	415,542
Non Regulated Energy [Member] | Non-regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross		322,855	319,611
Less: Accumulated depreciation and amortization		66,605	61,707
In-service utility plant, net		256,250	257,904
Construction work in progress		1,008	1,836
MGE [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, net		1,369,795	1,283,341
Construction work in progress		139,671	58,044
Total property, plant, and equipment		1,509,466	1,341,385
Assets held for sale		3,080	8,817
Allowance for Funds Used During Construction
AFUDC equity recognized		3,284	1,222	$ 1,207
MGE [Member] | Regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross		1,753,031	1,624,906
Less: Accumulated depreciation and amortization	[2],[3]	639,486	599,469
In-service utility plant, net		1,113,545	1,025,437
Construction work in progress	[2],[4]	138,663	56,208
MGE [Member] | Electric [Member] | Regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross	[2],[3]	1,310,438	1,209,353
MGE [Member] | Gas [Member] | Regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross		442,593	415,553
MGE [Member] | Non Regulated Energy [Member] | Non-regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross		322,855	319,611
Less: Accumulated depreciation and amortization		66,605	61,707
In-service utility plant, net		256,250	257,904
Construction work in progress		$ 1,008	$ 1,836

[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] (a ) As of December 31, 2018 and 2017 , MGE has classified $ 3.1 million and $ 8.8 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.

[3] (b) In April 2018, MGE, along with two other utilities, purchased the Forward Wind Energy Center (Forward Wind), which consists of 86 wind turbines located in Wisconsin. The aggregate purchase price was approximately $ 174 million, of which MGE's proportionate share is 12.8 %, or approximately $ 23 million.

[4] (c) In December 2017, the PSCW authorized construction of a 66 MW wind farm, consisting of 33 turbines, located near Saratoga, Iowa. MGE received specific approval to recover 100 % AFUDC on the project. As of December 31, 2018 , MG E has incurred $ 95.8 million of capital expenditures. After tax, MGE has recognized $ 2.5 million in AFUDC equity related to this project for the year ended December 31, 2018 . Construction of the project was completed in February 2019 .