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Summary of Significant Accounting Policies - Net Changes in Capitalized Exploratory Well Costs (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Increase (Decrease) in Capitalized Exploratory Well Costs that are Pending Determination of Proved Reserves [Roll Forward]      
Beginning of period $ 2,097 $ 2,097 $ 34,522
Exploratory well cost additions (pending determination of proved reserves) 10 0 51,803
Exploratory well cost reclassifications (successful determination of proved reserves) (571) 0 (84,228)
Exploratory well dry hole costs (unsuccessful in adding proved reserves) (1,536) 0 0
End of period $ 0 $ 2,097 $ 2,097