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Our people - Reportable Segment Distribution of Staff (Detail) - Employees
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of operating segments [line items]      
Average number of staff 132,823 133,903 134,428
Global Integrated Agencies [member]      
Disclosure of operating segments [line items]      
Average number of staff 82,295 83,015 81,537
Data investment management [member]      
Disclosure of operating segments [line items]      
Average number of staff 26,325 27,813 28,014
Public relations [member]      
Disclosure of operating segments [line items]      
Average number of staff 6,890 6,891 6,899
Specialist agencies [member]      
Disclosure of operating segments [line items]      
Average number of staff 17,313 16,184 17,978