Property, Plant, and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Property, Plant and Equipment [Line Items]
In-service utility plant, net	$ 1,189,077		$ 1,018,809
Construction work in progress	19,029		141,415
Total property, plant, and equipment	1,208,106		1,160,224
First Mortgage Bonds [Member] | 7.70%, 2028 Series
Plant related debt items
Long-term debt, gross	1,200	[1]	1,200	[1]
Regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross	1,480,928		1,304,984
Less: Accumulated depreciation and amortization	559,615		560,066
In-service utility plant, net	921,313		744,918
Construction work in progress	16,988		140,301
Electric [Member] | Regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross	1,110,953		953,290
Gas [Member] | Regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross	369,975		351,694
Non Regulated Energy [Member] | Non-regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross	313,152		311,742
Less: Accumulated depreciation and amortization	45,388		37,851
In-service utility plant, net	267,764		273,891
Construction work in progress	2,041		1,114
MGE [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, net	1,188,351		1,017,877
Construction work in progress	19,029		141,415
Total property, plant, and equipment	1,207,380		1,159,292
MGE [Member] | First Mortgage Bonds [Member] | 7.70%, 2028 Series
Plant related debt items
Long-term debt, gross	1,200	[1]	1,200	[1]
MGE [Member] | Regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross	1,480,957		1,305,012
Less: Accumulated depreciation and amortization	559,615		560,066
In-service utility plant, net	921,342		744,946
Construction work in progress	16,988		140,301
MGE [Member] | Electric [Member] | Regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross	1,110,970		953,307
MGE [Member] | Gas [Member] | Regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross	369,987		351,705
MGE [Member] | Non Regulated Energy [Member] | Non-regulated [Member]
Property, Plant and Equipment [Line Items]
In-service utility plant, gross	312,314		310,745
Less: Accumulated depreciation and amortization	45,305		37,814
In-service utility plant, net	267,009		272,931
Construction work in progress	$ 2,041		$ 1,114

[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 31,2014, approximately $334.8 million was available for the payment of dividends under this covenant.