Fair Value of Financial Instruments (Details-4) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance,		$ (41,463)	$ (48,354)	$ (42,026)	$ (50,305)
Realized and unrealized gains (losses):
Included in regulatory assets		4,130	2,748	4,693	4,701
Included in other comprehensive income		0	0	0	0
Included in earnings	[1]	82	(550)	(460)	(1,136)
Included in current assets		(297)	(222)	(470)	(97)
Purchases		6,032	6,186	11,866	12,182
Sales		0	0	0	0
Issuances		0	0	0	0
Settlements		(5,816)	(5,413)	(10,935)	(10,950)
Transfers in and/or out of level 3		0	0	0	0
Ending balance,		(37,332)	(45,605)	(37,332)	(45,605)
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]	60	(487)	(663)	(918)
Cost Of Gas Sold Expense [Member]
Realized and unrealized gains (losses):
Included in earnings	[1]	$ 22	$ (63)	$ 203	$ (218)

[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.