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Trade and Other Payables - Additional Information (Detail) - GBP (£)
Dec. 31, 2018
Dec. 31, 2017
Bottom of range [member] | Earnout agreements for acquisitions completed in the current year [member]    
Disclosure of trade and other payables [line items]    
Potential undiscounted amount of future payments £ 0 £ 0
Bottom of range [member] | All earn-out agreements [member]    
Disclosure of trade and other payables [line items]    
Potential undiscounted amount of future payments 0 0
Top of range [member] | Earnout agreements for acquisitions completed in the current year [member]    
Disclosure of trade and other payables [line items]    
Potential undiscounted amount of future payments 179,000,000 228,000,000
Top of range [member] | All earn-out agreements [member]    
Disclosure of trade and other payables [line items]    
Potential undiscounted amount of future payments £ 1,960,000,000 £ 1,910,000,000