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Intangible assets - Movement of Goodwill (Parenthetical) (Detail)
£ in Millions
 12 Months Ended
Dec. 31, 2019
GBP (£)
Disclosure of reconciliation of changes in goodwill [line items]  
Impairment on Classification As Held for Sale for Kantar Group £ 94.5 [1]
Accumulated impairment [member]  
Disclosure of reconciliation of changes in goodwill [line items]  
Impairment on Classification As Held for Sale for Kantar Group £ 70.9
[1] Goodwill impairment of £94.5 million arose from the assessment of fair value less costs to sell under IFRS 5.