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Goodwill, acquisitions and disposals - Additional Information (Detail) - GBP (£)
£ in Millions
 6 Months Ended 12 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
Disclosure of Goodwill acquisitions and disposals [Line Items]      
(Decrease) increase in Goodwill by subsidiary undertaking £ (2,014.3) £ 49.0  
Goodwill impairment 2,484.7 [1]   £ 47.7 [2]
Goodwill foreign currency translation difference £ 473.3    
Projection period for determining impairment of goodwill 5 years    
Global Integrated Agencies Segment [member]      
Disclosure of Goodwill acquisitions and disposals [Line Items]      
Goodwill impairment £ 1,606.9    
Public Relation Agencies Segment [member]      
Disclosure of Goodwill acquisitions and disposals [Line Items]      
Goodwill impairment 142.7    
Specialist Agencies Segment [member]      
Disclosure of Goodwill acquisitions and disposals [Line Items]      
Goodwill impairment £ 735.1    
Bottom of range [member]      
Disclosure of Goodwill acquisitions and disposals [Line Items]      
Revenue less pass through costs compound annual growth rate 2.50%    
Average operating margins 6.60%    
Top of range [member]      
Disclosure of Goodwill acquisitions and disposals [Line Items]      
Revenue less pass through costs compound annual growth rate 5.00%    
Average operating margins 19.00%    
First Step [member]      
Disclosure of Goodwill acquisitions and disposals [Line Items]      
Pre-tax discount rate 13.50%   8.50%
Pre-tax discount rate For smaller CGUs that operate primarily in a particular region subject to higher risk, the higher of 13.5% or 100 basis points above the regional discount rate was used in the first step.    
Assumed annual long-term growth rate 0.00%   3.00%
First Step [member] | Global Networks [member]      
Disclosure of Goodwill acquisitions and disposals [Line Items]      
Pre-tax discount rate 13.50%    
Second Step [member]      
Disclosure of Goodwill acquisitions and disposals [Line Items]      
Assumed annual long-term growth rate 2.00%   3.00%
Management forecasts for a projection period five-years    
Pre-tax discount rate for global CGUs [member]      
Disclosure of Goodwill acquisitions and disposals [Line Items]      
Pre-tax discount rate 12.50%    
Bottom of range of Pre-tax discount rates for Global CGUs [member].      
Disclosure of Goodwill acquisitions and disposals [Line Items]      
Pre-tax discount rate     6.30%
Top of range of Pre-tax discount rates for Global CGUs [member]      
Disclosure of Goodwill acquisitions and disposals [Line Items]      
Pre-tax discount rate     7.40%
Range Of Pretax Discount Rates For Regional Specific Cash Generating Units [member] | Bottom of range [member]      
Disclosure of Goodwill acquisitions and disposals [Line Items]      
Pre-tax discount rate 10.80%   4.10%
Range Of Pretax Discount Rates For Regional Specific Cash Generating Units [member] | Top of range [member]      
Disclosure of Goodwill acquisitions and disposals [Line Items]      
Pre-tax discount rate 19.30%   13.60%
[1] Figures have been restated as described in the accounting policies.
[2] Figures for the period ended 30 June 2020 have been restated, as described in the accounting policies.