Derivative and Hedging Instruments (Details-1) $ in Thousands		9 Months Ended		12 Months Ended
		Sep. 30, 2018 USD ($) Dth MWh MW		Dec. 31, 2017 USD ($) Dth MWh MW	Jun. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Derivative fair values [Abstract]
Receivable, margin account balance, net of collateral posted against derivative positions		$ 780		$ 2,507
Other Current Assets [Member]
Derivative fair values [Abstract]
Derivative fair value, net		894		806	$ 640	$ 874	$ 618	$ 230
Commodity Contracts And Financial Transimission Rights [Member]
Derivative fair values [Abstract]
Derivative fair value, net		$ 1,600		$ 200
Commodity Derivative Contracts [Member]
Gross Notional Volume of Open Derivatives
Notional amount, energy measure (in MWh) | MWh		482,450		552,310
Notional amount, decatherm measure (in Dth) | Dth		8,557,000		5,460,000
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis		$ 1,449		$ 676
Liability Derivatives, fair value, gross basis		287		793
Commodity Derivative Contracts [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	[1]	1,363
Liability Derivatives, fair value, gross basis		232	[1]	100
Derivative, right to reclaim collateral (receivable)		0
Commodity Derivative Contracts [Member] | Other Deferred Charges [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	[1]	86
Liability Derivatives, fair value, gross basis	[1]	$ 55
Commodity Derivative Contracts [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	[1],[2]			566
Liability Derivatives, fair value, gross basis	[1],[2]			603
Derivative, right to reclaim collateral (receivable)				100
Commodity Derivative Contracts [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	[1]			110
Liability Derivatives, fair value, gross basis	[1]			$ 190
Energy Related Commodity Contract [Member] | Cash Flow Hedging [Member]
Derivatives Fair Value [Line Items]
Maximum term of derivative hedging contract		4 years
Financial Transmission Rights [Member]
Gross Notional Volume of Open Derivatives
Notional amount, power measure (in MW) | MW		3,681		2,226
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis		$ 478		$ 329
Financial Transmission Rights [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis		478		329
Liability Derivatives, fair value, gross basis		$ 0		$ 0
PPA [Member]
Gross Notional Volume of Open Derivatives
Notional amount, power measure (in MW) | MW		2,200		2,650
Derivative fair values [Abstract]
Derivative fair value, net		$ (36,300)		$ (42,200)
Liability Derivatives, fair value, gross basis		36,270		42,170
PPA [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis		9,350		8,180
PPA [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis		$ 26,920		$ 33,990

[1] As of December 31, 2017 , collateral of $0.1 million was posted against and netted with derivative liability positions on the consolidated balance sheets. No collateral was posted against derivative positions a s of September 30, 2018 .

[2] As of December 31, 2017 , MGE presented $0.1 million as other current assets on the consolidated balance sheets.