Fair Value of Financial Instruments (Details-4) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance,		$ (4,533)	$ (21,023)	$ (14,055)	$ (26,456)
Realized and unrealized gains (losses):
Included in regulatory assets		0	5,296	0	10,729
Included in regulatory liability		8,122	0	17,644	0
Included in other comprehensive income		0	0	0	0
Included in earnings	[1]	2,476	(1,524)	2,686	(4,687)
Included in current assets		0	(37)	175	(101)
Purchases		6,638	5,505	18,899	15,837
Sales		0	0	0	0
Issuances		0	0	0	0
Settlements		(9,114)	(3,944)	(21,760)	(11,049)
Ending balance,		$ 3,589	$ (15,727)	$ 3,589	$ (15,727)

[1] MGE's exchange-traded derivative contracts, over-the-counter party transactions, purchased power agreement, and FTRs are subject to regulatory deferral. These derivatives are therefore marked to fair value and are offset in the financial statements with a corresponding regulatory asset or liability.