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Property, Plant and Equipment (Tables)
 6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Oil And Gas Property [Abstract]    
Summary of Oil and Natural Gas Property Costs Not Being Amortized  

The following table sets forth a summary of the Company’s oil and natural gas property costs not being amortized at December 31, 2017, by the year in which such costs were incurred (in thousands):

 

            Year Ended December 31,  
     Total      2017      2016      2015      2014 and Prior  

Acquisition

    $ 23,871      $ —        $ 3,845      $ 4,089      $ 15,937  

Exploration

      48,131        27,137        7,174        2,621        11,199  
  

 

 

     

 

 

     

 

 

     

 

 

     

 

 

  

Total unproved properties, not subject to amortization

    $ 72,002      $ 27,137      $ 11,019      $ 6,710      $ 27,136  
  

 

 

     

 

 

     

 

 

     

 

 

     

 

 

  
Schedule of Asset Retirement Obligations

The discounted asset retirement obligations included on the condensed consolidated balance sheets in current and non-current liabilities and the changes to that liability during the six months ended June 30, 2018 were as follows (in thousands):

 

Asset retirement obligations at January 1, 2018

    $ 214,733  

Fair value of asset retirement obligations assumed

      220,637  

Obligations settled

      (43,896 ) 

Accretion expense

      14,252  

Obligations incurred

      120  

Changes in estimate

      8,532  
  

 

 

  

Asset retirement obligations at June 30, 2018

    $ 414,378  

Less: Current portion

      94,334  
  

 

 

  

Long-term portion

    $ 320,044  
  

 

 

  

The discounted asset retirement obligations included in the consolidated balance sheets in current and non-current liabilities, and the changes in that liability during the each of the years ended December 31, were as follows (in thousands):

 

     Year Ended December 31,  
     2017      2016  

Asset retirement obligations at January 1

    $ 220,049      $ 226,690  

Fair value of asset retirement obligations acquired

      699        6,445  

Obligations settled

      (32,573 )       (23,689 ) 

Accretion expense

      19,295        21,829  

Obligations incurred

      4,213        1,014  

Changes in estimate(1)

      3,050        (12,240 ) 
  

 

 

     

 

 

  

Asset retirement obligations at December 31

    $ 214,733      $ 220,049  

Less: Current portion at December 31

      (39,741 )       (33,556 ) 
  

 

 

     

 

 

  

Noncurrent portion at December 31

    $ 174,992      $ 186,493  
  

 

 

     

 

 

  

 

(1)

The reduction during the year ended December 31, 2016 was primarily attributable to a reduction in service costs.