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Average number of employees - Summary of average number of employees (Parenthetical) (Detail) - Employee
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of average number of employees [Line Items]      
Average number of employees 46,007 47,458 46,807
Singapore [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 430 422 434
India [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 272 288 310