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Costs of services and general and administrative costs - Additional Information (Detail) - GBP (£)
£ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Dec. 31, 2018
Jun. 30, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure Of Information About Operating Cost [line items]          
Credits relating to the release of excess provisions and other balances     £ 26.9 £ 25.6 £ 40.9
Goodwill impairment     47.7 183.9 [1] 27.1 [1]
Investment write-downs     7.5 2.0 91.7
Restructuring and transformation costs     153.5 265.5 56.8
Gains on disposal of investments and subsidiaries     40.4 237.9 98.7
Litigation settlement     £ 16.8    
Lease Term     15 years    
Gain on sale of freehold property in New York     £ 7.9    
Proceeds from sale of freehold property     159.0    
Impairment charge on intangible assets     26.5 89.1 6.0
Investor day in december two thousand and eighteen restructuring and transformation plan [member]          
Disclosure Of Information About Operating Cost [line items]          
Restructuring and transformation costs £ 212.3        
Restructuring and transformation costs in relation to the continuing restructuring plan     121.1    
Transformation costs     32.4    
Costs arising from a structural assessment of operations [member]          
Disclosure Of Information About Operating Cost [line items]          
Restructuring and transformation costs   £ 53.2 116.3 179.7 33.7
Transformation cost [member]          
Disclosure Of Information About Operating Cost [line items]          
Restructuring and transformation costs     37.2 85.8  
IT services and infrastructure cost [member]          
Disclosure Of Information About Operating Cost [line items]          
Restructuring and transformation costs         12.8
comScore Inc. [member]          
Disclosure Of Information About Operating Cost [line items]          
Investment write-downs         53.1
Litigation settlement     16.8    
Globant S.A. [member]          
Disclosure Of Information About Operating Cost [line items]          
Gains on disposal of investments and subsidiaries     28.6 185.3  
Asatsu-DK Inc. [member]          
Disclosure Of Information About Operating Cost [line items]          
Gains on disposal of investments and subsidiaries         92.3
VMLY&R [member]          
Disclosure Of Information About Operating Cost [line items]          
Goodwill impairment       148.0  
Other business [member]          
Disclosure Of Information About Operating Cost [line items]          
Goodwill impairment     £ 47.7 £ 183.9 £ 27.1
[1] 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.