Fair Value of Financial Instruments (Details-4) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ (64,628)		$ (72,346)		$ (40,661)
Realized and unrealized gains (losses):
Included in regulatory liabilities (assets)	10,642		7,718		(31,685)
Included in other comprehensive income	0		0		0
Included in earnings	5,129	[1]	(2,618)	[1]	(5,005)	[1]
Included in current assets	0		(108)		0
Purchases	26,382		23,726		13,370
Sales	(125)		(2)		92
Issuances	0		0		0
Settlements	(31,386)		(20,998)		(8,457)
Transfers in and/or out of Level 3	0		0		0
Ending balance	(53,986)		(64,628)		(72,346)
Total gains (losses) included in earnings attributed to the change in unrealized gains (losses) related to assets and liabilities	0	[1]	0	[1]	0	[1]
Purchased Power Expense [Member]
Realized and unrealized gains (losses):
Included in earnings	5,137	[1]	(2,618)	[1]	(5,005)	[1]
Cost Of Gas Sold Expense [Member]
Realized and unrealized gains (losses):
Included in earnings	$ (8)	[1]	$ 0	[1]	$ 0	[1]

[1] MGE's exchange-traded derivative contracts, over-the-counter party transactions, ten-year 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.