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Asset Retirement Obligations - Schedule of Changes in Asset Retirement Obligations (Details)
$ in Thousands
6 Months Ended
Jun. 30, 2019
USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
December 31, 2018 $ 52,449
Liabilities incurred during period 1,019
Liabilities settled during period (139)
Accretion expense during period 1,928
Revisions to estimates 827
June 30, 2019 $ 56,084