Fair Value of Financial Instruments (Details-4) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance		$ (50,306)	$ (53,501)	$ (53,986)
Realized and unrealized gains (losses):
Included in regulatory liabilities		8,281	3,195	484
Included in other comprehensive income		0	0	0
Included in earnings	[1]	(3,718)	(5,347)	(6,635)
Included in current assets		(9)	(142)	0
Purchases		24,181	23,346	23,052
Sales		0	0	0
Issuances		0	0	0
Settlements		(20,455)	(17,857)	(16,416)
Transfers in and/or out of Level 3		0	0	0
Ending balance		(42,026)	(50,306)	(53,501)
Total gains (losses) included in earnings attributed to the change in unrealized gains (losses) related to assets and liabilities	[1]	0	0	0
Purchased Power Expense [Member]
Realized and unrealized gains (losses):
Included in earnings	[1]	(3,314)	(5,262)	(6,663)
Cost Of Gas Sold Expense [Member]
Realized and unrealized gains (losses):
Included in earnings	[1]	$ (404)	$ (85)	$ 28

[1] MGE's exchange-traded derivative contracts, over-the-count er 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.