Fair Value of Financial Instruments (Details-4) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance,		$ (56,194)	$ (49,349)	$ (53,986)	$ (64,628)
Realized and unrealized gains (losses):
Included in regulatory liabilities (assets)		(888)	(3,752)	(3,096)	11,528
Included in other comprehensive income		0	0	0	0
Included in earnings	[1]	(2,285)	(1,419)	(4,497)	5,837
Included in current assets		0	0	0	(89)
Purchases		6,364	6,371	17,497	19,900
Sales		0	0	34	(60)
Issuances		0	0	0	0
Settlements		(4,079)	(4,952)	(13,034)	(25,589)
Transfers in and/or out of level 3		0	0	0	0
Ending balance,		(57,082)	(53,101)	(57,082)	(53,101)
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]	(2,302)	(1,411)	(4,515)	5,845
Cost Of Gas Sold Expense [Member]
Realized and unrealized gains (losses):
Included in earnings	[1]	$ 17	$ (8)	$ 18	$ (8)

[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.