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Intangible assets - Cash-generating Units with Significant Goodwill (Detail) - GBP (£)
£ in Millions
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of information for cash-generating units [line items]      
Goodwill £ 13,202.8 £ 12,952.9 [1] £ 13,214.3
GroupM [member]      
Disclosure of information for cash-generating units [line items]      
Goodwill 2,942.9 2,906.7  
Kantar [member]      
Disclosure of information for cash-generating units [line items]      
Goodwill 2,522.9 2,518.2  
Wunderman [member]      
Disclosure of information for cash-generating units [line items]      
Goodwill 1,581.2 1,514.5  
VMLY&R [member]      
Disclosure of information for cash-generating units [line items]      
Goodwill 930.4 1,091.8  
Other [member]      
Disclosure of information for cash-generating units [line items]      
Goodwill £ 5,225.4 £ 4,921.7  
[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.