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Intangible Assets - Additional Information (Detail) - GBP (£)
£ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2023
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Disclosure of detailed information about intangible assets [line items]        
Other intangible assets £ 849.9 £ 849.9 £ 1,451.9 £ 1,359.5
Amortisation and impairment of acquired intangible assets   727.9 61.9 97.8
Goodwill impairment   63.6 £ 37.9 1.8
Global Integrated Agencies        
Disclosure of detailed information about intangible assets [line items]        
Impairment loss recognised in profit or loss, intangible assets other than goodwill   40.3    
Specialist Agencies        
Disclosure of detailed information about intangible assets [line items]        
Impairment loss recognised in profit or loss, intangible assets other than goodwill   £ 23.3    
First Step        
Disclosure of detailed information about intangible assets [line items]        
Assumed annual long-term growth rate 0.00% 0.00% 0.00%  
Discount rate applied to cash flow projections 14.70% 14.70% 15.50%  
Range of Pre-tax Discount Rates for Global Cash Generating Units        
Disclosure of detailed information about intangible assets [line items]        
Discount rate applied to cash flow projections 13.70% 13.70% 14.50%  
Range of Pre-tax Discount Rates for Geographic Regions        
Disclosure of detailed information about intangible assets [line items]        
Discount rate applied to cash flow projections 10000000000.00% 10000000000.00%    
Second Step        
Disclosure of detailed information about intangible assets [line items]        
Assumed annual long-term growth rate 2.00% 2.00% 2.00%  
Management forecasts for a projection period   five years    
Bottom of Range | Range of Pre-tax Discount Rates for Geographic Regions        
Disclosure of detailed information about intangible assets [line items]        
Discount rate applied to cash flow projections 12.60% 12.60% 14.00%  
Top of Range | Range of Pre-tax Discount Rates for Geographic Regions        
Disclosure of detailed information about intangible assets [line items]        
Discount rate applied to cash flow projections 28.40% 28.40% 22.60%  
Brand names        
Disclosure of detailed information about intangible assets [line items]        
Other intangible assets £ 134.6 £ 134.6 £ 142.3  
Impairment loss recognised in profit or loss, intangible assets other than goodwill   650.1 1.4 £ 47.9
Customer-related intangibles        
Disclosure of detailed information about intangible assets [line items]        
Other intangible assets 108.2 108.2 120.3  
Other acquired intangible assets        
Disclosure of detailed information about intangible assets [line items]        
Other intangible assets 101.4 £ 101.4 £ 26.1  
Wunderman Thompson        
Disclosure of detailed information about intangible assets [line items]        
Amortisation and impairment of acquired intangible assets 430.8      
VMLY&R        
Disclosure of detailed information about intangible assets [line items]        
Amortisation and impairment of acquired intangible assets £ 202.3