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Asset Retirement Obligations - Schedule of Changes in Asset Retirement Obligations (Details)
$ in Thousands
6 Months Ended
Jun. 30, 2018
USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
December 31, 2017 $ 48,799
Liabilities incurred during period 4,008
Liabilities settled during period (1)
Accretion expense during period 1,314
Liabilities held for sale(2) 84
Asset Retirement Obligation, Liabilities Held for Sale (4,167)
June 30, 2018 $ 50,037