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Asset Retirement Obligations - Schedule of Changes in Asset Retirement Obligations (Details)
$ in Thousands
9 Months Ended
Sep. 30, 2018
USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
December 31, 2017 $ 48,799
Liabilities incurred during period 5,509
Liabilities settled during period (4,904)
Accretion expense during period 1,975
Liabilities held for sale 83
September 30, 2018 $ 51,462