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Intangible assets - Additional Information (Detail) - GBP (£)
£ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of detailed information about intangible assets [line items]      
Other Intangible Assets £ 1,842.0 £ 2,018.4 [1] £ 2,217.3
Amortisation and impairment of acquired intangible assets 280.0 195.1  
Impairment charge on intangible assets £ 126.1 £ 6.0  
Pre-tax discount rate 9.00% 8.50%  
Assumed annual long-term growth rate 3.00% 3.00%  
Goodwill impairment charge £ 183.9 £ 27.1 27.0
Discount Rates [member] | VMLY&R [member]      
Disclosure of detailed information about intangible assets [line items]      
Percentage of movement in discount rate 0.10%    
Increase or decrease the impairment charge £ 23.0    
Terminal Growth Rate [member] | VMLY&R [member]      
Disclosure of detailed information about intangible assets [line items]      
Increase or decrease the impairment charge £ 19.0    
Percentage Of Movement In Terminal growth rate 0.10%    
Terminal margin [member] | VMLY&R [member]      
Disclosure of detailed information about intangible assets [line items]      
Increase or decrease the impairment charge £ 8.0    
Percentage Of Movement In Terminal Margin 0.10%    
VMLY&R [member]      
Disclosure of detailed information about intangible assets [line items]      
Pre-tax discount rate 8.70% 8.50%  
Impairment loss recoverable amount £ 1,327.3    
Other business [member]      
Disclosure of detailed information about intangible assets [line items]      
Goodwill impairment charge 35.9    
Advertising and media investment management [member]      
Disclosure of detailed information about intangible assets [line items]      
Impairment charge on intangible assets 70.6    
Goodwill impairment charge 148.0 £ 19.5 20.9
Advertising and media investment management [member] | VMLY&R [member]      
Disclosure of detailed information about intangible assets [line items]      
Goodwill impairment charge 148.0    
Data investment management [member]      
Disclosure of detailed information about intangible assets [line items]      
Impairment charge on intangible assets 38.2    
Goodwill impairment charge 0.9    
Brand consulting, health & wellness and specialist communications [member]      
Disclosure of detailed information about intangible assets [line items]      
Impairment charge on intangible assets 17.3    
Goodwill impairment charge £ 35.0   £ 6.1
Top of range [member]      
Disclosure of detailed information about intangible assets [line items]      
Management forecasts for a projection period Five years    
Associates [member]      
Disclosure of detailed information about intangible assets [line items]      
Amortisation and impairment of acquired intangible assets £ 4.2 5.7  
Brand names [member]      
Disclosure of detailed information about intangible assets [line items]      
Other Intangible Assets 361.2 445.6  
Impairment charge on intangible assets 58.6    
Customer-related intangibles [member]      
Disclosure of detailed information about intangible assets [line items]      
Other Intangible Assets 220.6 360.9  
Other acquired intangible assets [member]      
Disclosure of detailed information about intangible assets [line items]      
Other Intangible Assets 13.0 £ 22.6  
Customer relationships [member]      
Disclosure of detailed information about intangible assets [line items]      
Impairment charge on intangible assets £ 67.5    
[1] Prior year figures have been restated for the impact of the adoption of IFRS 15 Revenue from Contracts with Customers, as described in the accounting policies.