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Accounting Policies Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]      
Beginning balance $ 3,506 $ 3,319  
Liabilities settled 0 (64)  
Accretion expense 266 251 $ 232
Ending balance $ 3,772 $ 3,506 $ 3,319