XML 61 R49.htm IDEA: XBRL DOCUMENT v3.8.0.1
Asset Retirement Obligations - Schedule of Changes in Asset Retirement Obligations (Details)
$ in Thousands
9 Months Ended
Sep. 30, 2017
USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
Balance at December 31, 2016 $ 49,687
Liabilities incurred during period 1,333
Liabilities settled during period (103)
Accretion expense during period 1,998
Revisions to estimates (215)
Balance at September 30, 2017 $ 52,700