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Utility Plant and Intangible Assets 3 (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Reconciliation of the beginning and ending aggregate carrying amount of the asset retirement obligations    
Obligation at the beginning of the period $ 4,157 $ 3,234
Additional liabilities incurred 121 7
Liabilities settled (112)  
Accretion 227 209
Obligation at the end of the period $ 4,393 4,157
Asset Retirement Obligation, Revision of Estimate   $ 707