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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Changes in ARO liability          
Asset retirement obligations, beginning of period     $ 21,285 $ 17,422  
Additional liabilities incurred     2,229 1,196  
Liabilities acquired     270 2,411  
Liabilities settled     (1,474) (689)  
Accretion expense $ 387 $ 357 1,107 1,030  
Revisions in estimated liabilities     568 4  
Asset retirement obligations, end of period 23,985 21,374 23,985 21,374  
Less current portion 88 1,392 88 1,392  
Asset retirement obligations - long-term $ 23,897 $ 19,982 $ 23,897 $ 19,982 $ 20,122