Revenue (Details-1) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended			12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017		Jun. 30, 2018	Jun. 30, 2017		Dec. 31, 2017
Total Operating Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	$ 124,262	$ 126,463	[1]	$ 281,894	$ 283,286	[1]
Electric [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	99,067	102,306		193,458	200,630
Electric [Member] | Residential [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	32,217	31,634		65,476	64,987
Electric [Member] | Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	50,174	54,721	[1]	97,626	104,885	[1]
Electric [Member] | Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	4,630	4,516		7,525	8,731
Electric [Member] | Other-retail/municipal
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	8,527	9,661		16,814	18,682
Electric [Member] | Total Retail [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	95,548	100,532		187,441	197,285
Electric [Member] | Sales To The Market [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	2,795	1,050		4,829	2,022
Electric [Member] | Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	454	562		924	991
Electric [Member] | Adjustments To Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	270	162		264	332
Gas [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	24,980	24,081		87,745	82,507
Gas [Member] | Residential [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	15,620	15,140		52,171	49,043
Gas [Member] | Commercial/Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	8,206	7,842	[1]	33,158	30,890	[1]
Gas [Member] | Total Retail [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	23,826	22,982		85,329	79,933
Gas [Member] | Gas Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	1,053	991		2,189	2,341
Gas [Member] | Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	101	108		227	233
Non Regulated Energy [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	$ 215	$ 76		$ 691	$ 149
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.