Derivative and Hedging Instruments (Details-1) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) Dth1 MWh MW		Dec. 31, 2020 USD ($) Dth1 MWh MW		Jun. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Derivative fair values [Abstract]
Collateral posted against derivative positions	$ 9,200		$ 0
Other Current Assets [Member]
Derivative fair values [Abstract]
Derivative fair value, net	766		1,162		$ 520	$ 983	$ 940	$ 1,100
Commodity Contracts And Financial Transimission Rights [Member]
Derivative fair values [Abstract]
Derivative fair value, net	$ 13,800		$ 200
Commodity Derivative Contracts [Member]
Gross Notional Volume of Open Derivatives
Notional amount, energy measure (in MWh) | MWh	290,520		259,080
Notional amount, decatherm measure (in Dth) | Dth1	9,050,000		6,030,000
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	$ 13,985		$ 806
Liability Derivatives, fair value, gross basis	375		632
Collateral posted against derivative positions	9,235		0
Commodity Derivative Contracts [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	13,426	[1]	617	[2]
Liability Derivatives, fair value, gross basis	375	[1]	593	[2]
Commodity Derivative Contracts [Member] | Other Deferred Charges [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	559	[1]	189	[2]
Liability Derivatives, fair value, gross basis	0	[1]	$ 39	[2]
Commodity Derivative Contracts [Member] | Cash Flow Hedging [Member]
Derivative fair values [Abstract]
Collateral posted against derivative positions	$ 9,200
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	3,280		2,869
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	$ 225
Liability Derivatives, fair value, gross basis			$ 23
Collateral posted against derivative positions	0
Financial Transmission Rights [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	225
Liability Derivatives, fair value, gross basis	$ 0
Financial Transmission Rights [Member] | Other Current Liabilities [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis			0
Liability Derivatives, fair value, gross basis			$ 23
PPA [Member]
Gross Notional Volume of Open Derivatives
Notional amount, power measure (in MW) | MW	400		850
Derivative fair values [Abstract]
Derivative fair value, net	$ (200)		$ (14,100)
Liability Derivatives, fair value, gross basis	160		14,140
PPA [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	160		10,160
PPA [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	$ 0		$ 3,980

[1] As of September 30, 2021, MGE received collateral of $9.2 million from counterparties under a master netting agreement for outstanding exchange traded derivative positions. The fair value of the derivative asset disclosed in this table has not been reduced for the collateral received.

[2] No collateral was posted against derivative positions as of December 31, 2020.