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Related party transactions - Summary of compensation paid or payable to key management (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of transactions between related parties [abstract]    
Salaries and other short-term employee benefits $ 9,515 $ 8,152
Defined benefit pension plan 754 1,350
Share based payments 5,920 5,326
Compensation for key management personnel $ 16,189 $ 14,828