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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, 2018
Dec. 31, 2017
Disclosure of transactions between related parties [abstract]    
Salaries and other short-term employee benefits $ 6,191 $ 8,908
Defined benefit pension plan 268 754
Share based payments 2,632 5,920
Termination benefits 1,762 607
Compensation for key management personnel $ 10,853 $ 16,189