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Average number of employees - Summary of average number of employees (Parenthetical) (Detail) - Employee
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of average number of employees [Line Items]      
Average number of employees 46,807 51,029 54,938
Singapore [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 434 262  
India [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 310 280