Derivative and Hedging Instruments (Details-1) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2020 USD ($)		Sep. 30, 2020 USD ($) Dth MWh MW		Dec. 31, 2019 USD ($) Dth MWh MW	Jun. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Derivative fair values [Abstract]
Collateral posted against derivative positions						$ 1,500
Collateral posted against derivative positions		$ 700		$ 700
Other Current Assets [Member]
Derivative fair values [Abstract]
Derivative fair value, net		983		983		1,100	$ 940	$ 1,031	$ 878	$ 377
Commodity Contracts And Financial Transimission Rights [Member]
Derivative fair values [Abstract]
Derivative fair value, net		2,000		$ 2,000		$ (1,400)
Commodity Derivative Contracts [Member]
Gross Notional Volume of Open Derivatives
Notional amount, energy measure (in MWh) | MWh				330,400		417,840
Notional amount, decatherm measure (in Dth) | Dth				8,495,000		6,605,000
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis		2,501		$ 2,501		$ 230
Liability Derivatives, fair value, gross basis		558		558		1,738
Collateral posted against derivative positions		0		0		1,546
Collateral posted against derivative positions		650		650		0
Commodity Derivative Contracts [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis		2,248	[1]	2,248	[1]	100
Liability Derivatives, fair value, gross basis	[1]	546		546
Commodity Derivative Contracts [Member] | Other Deferred Charges [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	[1]	253		253
Liability Derivatives, fair value, gross basis	[1]	12		12
Commodity Derivative Contracts [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	[2],[3]					157
Liability Derivatives, fair value, gross basis	[2],[3]					1,521
Commodity Derivative Contracts [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	[2],[3]					73
Liability Derivatives, fair value, gross basis	[2],[3]					217
Commodity Derivative Contracts [Member] | Cash Flow Hedging [Member]
Derivative fair values [Abstract]
Collateral posted against derivative positions						$ 1,500
Collateral posted against derivative positions		$ 700		$ 700
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				4,589		2,750
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis		$ 20		$ 20		$ 73
Collateral posted against derivative positions		0		0		0
Financial Transmission Rights [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis		20		20		73
Liability Derivatives, fair value, gross basis		0		$ 0		$ 0
PPA [Member]
Gross Notional Volume of Open Derivatives
Notional amount, power measure (in MW) | MW				1,000		1,450
Derivative fair values [Abstract]
Derivative fair value, net		(16,000)		$ (16,000)		$ (25,400)
Liability Derivatives, fair value, gross basis		15,980		15,980		25,440
Collateral posted against derivative positions		0		0		0
PPA [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis		9,640		9,640		10,100
PPA [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis		$ 6,340		$ 6,340		$ 15,340

[1] As of September 30, 2020, collateral of $0.7 million was posted against derivative positions owed to counterparties under a master netting agreement on the consolidated balance sheet.

[2] As of December 31, 2019, MGE posted $0.1 million as other current assets on the consolidated balance sheets.

[3] As of December 31, 2019, collateral of $1.5 million was posted against and netted with derivative liability positions on the consolidated balance sheets.