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Our people - Reportable Segment Distribution of Staff (Detail) - Employees
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of operating segments [line items]      
Average number of staff 133,903 134,428 132,657
Advertising and media investment management [member]      
Disclosure of operating segments [line items]      
Average number of staff 55,421 56,789 55,120
Data investment management [member]      
Disclosure of operating segments [line items]      
Average number of staff 28,309 28,629 29,279
Public relations & public affairs [member]      
Disclosure of operating segments [line items]      
Average number of staff 9,048 9,082 9,054
Brand consulting, health & wellness and specialist communications [member]      
Disclosure of operating segments [line items]      
Average number of staff 41,125 39,928 39,204