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Goodwill and acquisitions - Additional Information (Detail) - GBP (£)
£ in Millions
 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Disclosure Of Goodwill Acquisitions And Disposals [Line Items]    
Goodwill impairment £ 2,812.9 [1]
Assumed annual long-term growth rate   2.00%
Pre tax Discount Rate For Global Network CGUs [Member]    
Disclosure Of Goodwill Acquisitions And Disposals [Line Items]    
Pre-tax discount rate   12.50%
First Step [Member]    
Disclosure Of Goodwill Acquisitions And Disposals [Line Items]    
Assumed annual long-term growth rate   0.00%
Bottom of range [member] | Range Of Pre tax Discount Rates For Regional Specific Cash Generating Units [Member]    
Disclosure Of Goodwill Acquisitions And Disposals [Line Items]    
Pre-tax discount rate   10.80%
Top of range [member] | Range Of Pre tax Discount Rates For Regional Specific Cash Generating Units [Member]    
Disclosure Of Goodwill Acquisitions And Disposals [Line Items]    
Pre-tax discount rate   19.30%
Global Networks [Member] | First Step [Member]    
Disclosure Of Goodwill Acquisitions And Disposals [Line Items]    
Pre-tax discount rate   13.50%
Global Integrated Agencies Segment [Member]    
Disclosure Of Goodwill Acquisitions And Disposals [Line Items]    
Goodwill impairment   £ 1,820.1
Public Relation Agencies Segment [Member]    
Disclosure Of Goodwill Acquisitions And Disposals [Line Items]    
Goodwill impairment   157.1
Specialist Agencies Segment [Member]    
Disclosure Of Goodwill Acquisitions And Disposals [Line Items]    
Goodwill impairment   £ 835.7
[1] Figures have been restated as described in the accounting policies.