Revenue (Details-1) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended			12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017		Sep. 30, 2018	Sep. 30, 2017		Dec. 31, 2017
Total Operating Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	$ 137,795	$ 139,539	[1]	$ 419,689	$ 422,825	[1]
Electric [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	119,175	120,649		312,633	321,279
Electric [Member] | Residential [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	42,371	39,905		107,847	104,892
Electric [Member] | Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	61,638	64,036	[1]	159,264	168,921	[1]
Electric [Member] | Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	3,668	4,555		11,193	13,286
Electric [Member] | Other-retail/municipal
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	9,431	10,459		26,245	29,141
Electric [Member] | Total Retail [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	117,108	118,955		304,549	316,240
Electric [Member] | Sales To The Market [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	1,505	1,068		6,334	3,090
Electric [Member] | Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	482	445		1,406	1,436
Electric [Member] | Adjustments To Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	80	181		344	513
Gas [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	18,407	18,778		106,152	101,285
Gas [Member] | Residential [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	11,916	12,121		64,087	61,164
Gas [Member] | Commercial/Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	5,570	5,622	[1]	38,728	36,512	[1]
Gas [Member] | Total Retail [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	17,486	17,743		102,815	97,676
Gas [Member] | Gas Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	829	944		3,018	3,285
Gas [Member] | Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	92	91		319	324
Non Regulated Energy [Member]
Disaggregation of Revenue [Line Items]
Revenues recognized from contracts with customers	$ 213	$ 112		$ 904	$ 261
MGE [Member]
Disaggregation of Revenue [Line Items]
CNG Vehicle Fuel Servicing Revenue							$ 100

[1] (a) 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.