Fair Value of Financial Instruments (Details-4) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance,		$ (48,354)	$ (57,101)	$ (50,305)	$ (53,501)
Realized and unrealized gains (losses):
Included in regulatory liabilities		2,748	5,218	4,701	1,618
Included in other comprehensive income		0	0	0	0
Included in earnings	[1]	(550)	(1,809)	(1,136)	(3,972)
Included in current assets		(222)	0	(97)	0
Purchases		6,186	5,636	12,182	10,937
Sales		0	0	0	0
Issuances		0	0	0	0
Settlements		(5,413)	(3,827)	(10,950)	(6,965)
Transfers in and/or out of level 3		0	0	0	0
Ending balance,		(45,605)	(51,883)	(45,605)	(51,883)
Total gains (losses) included in earnings attributed to the change in unrealized gains (losses) related to assets and liabilities	[1]	0	0	0	0
Purchased Power Expense [Member]
Realized and unrealized gains (losses):
Included in earnings	[1]	(487)	(1,809)	(918)	(3,972)
Cost Of Gas Sold Expense [Member]
Realized and unrealized gains (losses):
Included in earnings	[1]	$ (63)	$ 0	$ (218)	$ 0

[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.