Revenue (Details-1) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Dec. 31, 2017
Total Operating Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	$ 157,632	$ 156,823	[1]
Electric [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	94,391	98,324
Electric [Member] | Residential [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	33,259	33,353
Electric [Member] | Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	47,452	50,164	[1]
Electric [Member] | Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	2,895	4,215
Electric [Member] | Other-retail/municipal
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	8,287	9,021
Electric [Member] | Total Retail [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	91,893	96,753
Electric [Member] | Sales To The Market [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	2,034	972
Electric [Member] | Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	470	429
Electric [Member] | Adjustments To Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	(6)	170
Gas [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	62,765	58,426
Gas [Member] | Residential [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	36,551	33,903
Gas [Member] | Commercial/Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	24,952	23,048	[1]
Gas [Member] | Total Retail [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	61,503	56,951
Gas [Member] | Gas Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	1,135	1,350
Gas [Member] | Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	127	125
Non Regulated Energy [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	$ 476	$ 73
MGE [Member]
Disaggregation of Revenue [Line Items]
CNG Vehicle Fuel Servicing Revenue				$ 100

[1] In 2017, MGE had less than $0.1 million of revenues related to CNG vehicle fuel servicing. No revenue was recognized for fuel servicing by MGE Energy in 2017.