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Intangible assets - Additional Information (Detail) - GBP (£)
£ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of detailed information about intangible assets [line items]      
Other Intangible Assets [1] £ 1,468.8 £ 1,842.0 £ 2,018.4
Amortisation and impairment of acquired intangible assets 121.5 201.8  
Impairment charge on intangible assets 26.5 89.1 6.0
Goodwill impairment charge [2] £ 47.7 £ 176.5 [3] £ 27.1 [3]
Pre-tax discount rate 8.50%    
Percentage of recoverable amount on long term growth of asset 25.00%    
First Step [Member]      
Disclosure of detailed information about intangible assets [line items]      
Assumed annual long-term growth rate 3.00% 3.00%  
Pre-tax discount rate 8.50% 9.00%  
Second Step [Member]      
Disclosure of detailed information about intangible assets [line items]      
Management forecasts for a projection period five years    
Assumed annual long-term growth rate 3.00% 3.00%  
Weighting Of The Regional Rates By Its Geographic Distribution Of Cash Flows [Member]      
Disclosure of detailed information about intangible assets [line items]      
Pre-tax discount rate 8.50% 9.00%  
VMLY&R [member]      
Disclosure of detailed information about intangible assets [line items]      
Goodwill impairment charge   £ 142.8  
Other business [member]      
Disclosure of detailed information about intangible assets [line items]      
Goodwill impairment charge   £ 33.7  
Top of range [member] | Range of Pre-tax discount rates for geographic regions [Member]      
Disclosure of detailed information about intangible assets [line items]      
Pre-tax discount rate 13.60% 16.30%  
Top of range [member] | Range of pre-tax discount rates for global cash generating units [Member]      
Disclosure of detailed information about intangible assets [line items]      
Pre-tax discount rate 7.40% 8.70%  
Bottom of range [member] | Range of Pre-tax discount rates for geographic regions [Member]      
Disclosure of detailed information about intangible assets [line items]      
Pre-tax discount rate 4.10% 6.20%  
Bottom of range [member] | Range of pre-tax discount rates for global cash generating units [Member]      
Disclosure of detailed information about intangible assets [line items]      
Pre-tax discount rate 6.30% 8.00%  
Associates [member]      
Disclosure of detailed information about intangible assets [line items]      
Amortisation and impairment of acquired intangible assets £ 5.6 £ 3.7  
Brand names [member]      
Disclosure of detailed information about intangible assets [line items]      
Other Intangible Assets 218.6 361.2  
Impairment charge on intangible assets 21.4    
Customer-related intangibles [member]      
Disclosure of detailed information about intangible assets [line items]      
Other Intangible Assets 100.6 220.6  
Other acquired intangible assets [member]      
Disclosure of detailed information about intangible assets [line items]      
Other Intangible Assets 4.4 £ 13.0  
Customer relationships [member]      
Disclosure of detailed information about intangible assets [line items]      
Impairment charge on intangible assets 5.1    
Brands With An Indefinite Life [Member]      
Disclosure of detailed information about intangible assets [line items]      
Impairment charge on intangible assets 13.2    
Public relations [Member]      
Disclosure of detailed information about intangible assets [line items]      
Impairment charge on intangible assets 13.2    
Global Integrated Agencies [Member]      
Disclosure of detailed information about intangible assets [line items]      
Impairment charge on intangible assets 13.0    
Specialist Agencies [Member]      
Disclosure of detailed information about intangible assets [line items]      
Impairment charge on intangible assets £ 0.3    
[1] Figures have been restated, as described in the accounting policies.
[2] Figures in 2018 have been restated, as described in the accounting policies.
[3] Prior year figures have been re-presented in accordance with IFRS 5 Non-current Assets Held for Sale and Discontinued Operations, as described in the accounting policies.