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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Asset Retirement Obligations [Roll Forward]    
Asset retirement obligation, beginning balance $ 26,886 $ 24,360
Liabilities incurred 145 1,303
Accretion expense 1,359 1,269
Liabilities settled (1,789) [1] (110)
Revisions in estimated cash flows 137 64
Asset retirement obligation, ending balance 26,738 $ 26,886
Blount Asbestos Removal [Member]    
Asset Retirement Obligations [Roll Forward]    
Liabilities settled $ (1,600)  
[1]

In 2017 , MGE removed asbestos at Blount for $1.6 million .