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Trade and other receivables - Summary of Aging Analysis of Invoice, Loss Allowance (Detail) - GBP (£)
£ in Millions
Dec. 31, 2018
Jan. 01, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of financial assets [Line items]          
Trade receivables £ 8,613.9   £ 8,390.1    
Trade receivables [member]          
Disclosure of financial assets [Line items]          
Trade receivables 8,062.2   7,889.7    
Loss allowance £ 116.6 £ 91.3 £ 91.3 £ 93.8 £ 85.4
Expected loss rate 1.40% 1.10%      
Trade receivables [member] | Cost [member]          
Disclosure of financial assets [Line items]          
Trade receivables £ 8,178.9 £ 7,981.0      
0-90 days [member] | Trade receivables [member]          
Disclosure of financial assets [Line items]          
Loss allowance £ 11.3 2.9      
Expected loss rate 0.10%        
0-90 days [member] | Trade receivables [member] | Cost [member]          
Disclosure of financial assets [Line items]          
Trade receivables £ 7,621.9 7,392.6      
91-180 days [member] | Trade receivables [member]          
Disclosure of financial assets [Line items]          
Loss allowance £ 3.8 £ 2.8      
Expected loss rate 1.00% 0.70%      
91-180 days [member] | Trade receivables [member] | Cost [member]          
Disclosure of financial assets [Line items]          
Trade receivables £ 381.5 £ 425.6      
181 days - 1 year [member] | Trade receivables [member]          
Disclosure of financial assets [Line items]          
Loss allowance £ 39.2 £ 24.2      
Expected loss rate 45.30% 30.80%      
181 days - 1 year [member] | Trade receivables [member] | Cost [member]          
Disclosure of financial assets [Line items]          
Trade receivables £ 86.5 £ 78.6      
Over 1 year [member] | Trade receivables [member]          
Disclosure of financial assets [Line items]          
Loss allowance £ 62.3 £ 61.4      
Expected loss rate 70.00% 73.00%      
Over 1 year [member] | Trade receivables [member] | Cost [member]          
Disclosure of financial assets [Line items]          
Trade receivables £ 89.0 £ 84.2