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Asset Retirement Obligations - Schedule of Changes in Asset Retirement Obligations (Details)
$ in Thousands
6 Months Ended
Jun. 30, 2017
USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
Balance at December 31, 2016 $ 49,687
Liabilities incurred during period 661
Liabilities settled during period (101)
Accretion expense during period 1,316
Revisions to estimates (214)
Balance at June 30, 2017 $ 51,349