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Asset Retirement Obligations (Summary of Changes In Asset Retirement Obligations) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Asset Retirement Obligations [Abstract]      
Balance at January 1 $ 18,612 $ 16,166 $ 14,846
Accretion 951 903 778
Additions     1,085
Acquisitions and dispositions (103) 1,544  
Revisions 703 (1) (543)
Balance at December 31 $ 20,163 $ 18,612 $ 16,166