Fair Value of Financial Instruments (Details-4) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance,		$ (37,332)	$ (45,605)	$ (42,026)	$ (50,305)
Realized and unrealized gains (losses):
Included in regulatory assets		2,018	2,846	6,711	7,547
Included in other comprehensive income		0	0	0	0
Included in earnings	[1]	26	(1,478)	(434)	(2,614)
Included in current assets		(25)	(14)	(496)	(111)
Purchases		5,736	6,299	17,602	18,481
Sales		0	0	0	0
Issuances		0	0	0	0
Settlements		(5,737)	(4,807)	(16,671)	(15,757)
Transfers in and/or out of level 3		0	0	0	0
Ending balance,		(35,314)	(42,759)	(35,314)	(42,759)
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]	26	(1,460)	(637)	(2,377)
Cost Of Gas Sold Expense [Member]
Realized and unrealized gains (losses):
Included in earnings	[1]	$ 0	$ (18)	$ 203	$ (237)

[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.