Asset Retirement Obligations (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015		Dec. 31, 2014
Asset Retirement Obligations [Roll Forward]
Asset retirement obligation, beginning balance		$ 19,744		$ 19,359
Liabilities incurred		2,380	[1]	68
Accretion expense		1,131		1,077
Liabilities settled		(124)		(343)
Revisions in estimated cash flows		1,229	[1]	(417)
Asset retirement obligation, ending balance		24,360		$ 19,744
Columbia Units [Member]
Asset Retirement Obligations [Roll Forward]
Liabilities incurred	[1]	2,300
Revisions in estimated cash flows	[1]	$ 1,300

[1] In the second quarter of 2015 , MGE recorded an obligation of $2.3 million for the fair value of its legal liability for AROs associated with the effect of the final Coal Combustion Residual Rule at Columbia. An additional $1.3 million was recorded in the fourth quarter, associated with this ARO, based on revised estimates.