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Revenue (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure Of Revenue [Line Items]      
Sale of crude oil $ 279,162 $ 145,193 $ 162,629
Sale of gas 50,960 47,477 47,061
Revenue $ 330,122 $ 192,670 $ 209,690