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Property, Plant and Equipment - Schedule of Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Oil And Gas Property [Abstract]              
Asset retirement obligations     $ 214,733 $ 220,049 $ 220,049 $ 226,690  
Fair value of asset retirement obligations assumed     220,637        
Obligations settled     (43,896)   (32,573) (23,689)  
Accretion expense $ 9,492 $ 5,321 14,252 $ 10,509 19,295 21,829 $ 19,395
Obligations incurred     120   4,213 1,014  
Changes in estimate     8,532   3,050 (12,240)  
Asset retirement obligations 414,378   414,378   214,733 220,049 $ 226,690
Current portion of asset retirement obligations 94,334   94,334   39,741 33,556  
Noncurrent portion $ 320,044   $ 320,044   174,992 186,493  
Fair value of asset retirement obligations acquired         $ 699 $ 6,445