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Sources of finance - Additional Information (Detail)
€ in Millions, £ in Millions, $ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2019
GBP (£)
Dec. 31, 2019
USD ($)
Dec. 31, 2019
EUR (€)
Dec. 31, 2019
AUD ($)
Dec. 31, 2018
GBP (£)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
GBP (£)
Dec. 31, 2016
GBP (£)
Disclosure of detailed information about financial instruments [line items]                
Total equity | £ £ 8,443.5       £ 9,806.6   £ 9,956.1 £ 9,761.7
Total committed facilities available | £ 6,309.8              
Undrawn committed credit facility | £ 2,005.6       2,074.7      
Borrowings | £ 4,272.9       6,217.9      
Issued capital and share premium [member]                
Disclosure of detailed information about financial instruments [line items]                
Total equity | £ 703.1       £ 703.0   £ 701.8  
US$ bonds due September 2022 [member]                
Disclosure of detailed information about financial instruments [line items]                
Total committed facilities available £ 377.0 $ 500            
Bonds interest rate 3.625% 3.625% 3.625% 3.625%        
US$ bonds due September 2024 [member]                
Disclosure of detailed information about financial instruments [line items]                
Total committed facilities available £ 565.5 $ 750            
Bonds interest rate 3.75% 3.75% 3.75% 3.75%        
US$ bonds due September 2042 [member]                
Disclosure of detailed information about financial instruments [line items]                
Total committed facilities available £ 70.0 $ 93            
Bonds interest rate 5.125% 5.125% 5.125% 5.125%        
US$ bonds due November 2043 [member]                
Disclosure of detailed information about financial instruments [line items]                
Total committed facilities available £ 165.8 $ 220            
Bonds interest rate 5.625% 5.625% 5.625% 5.625%        
Eurobonds (3m EURIBOR + 0.32%) due May 2020 [member]                
Disclosure of detailed information about financial instruments [line items]                
Total committed facilities available £ 211.6   € 250          
Floating basis 3m EURIBOR              
Adjustment to floating basis 0.32% 0.32% 0.32% 0.32%        
Euro bonds due November 2023 [member]                
Disclosure of detailed information about financial instruments [line items]                
Total committed facilities available £ 634.9   € 750          
Bonds interest rate 3.00% 3.00% 3.00% 3.00%        
Euro bonds due September 2026 [member]                
Disclosure of detailed information about financial instruments [line items]                
Total committed facilities available £ 634.9   € 750          
Bonds interest rate 2.25% 2.25% 2.25% 2.25%        
Euro bonds due March 2030 [member]                
Disclosure of detailed information about financial instruments [line items]                
Total committed facilities available £ 507.9   € 600          
Bonds interest rate 1.625% 1.625% 1.625% 1.625%        
Eurobonds Due March 2025 [member]                
Disclosure of detailed information about financial instruments [line items]                
Total committed facilities available £ 423.3   € 500          
Bonds interest rate 1.375% 1.375% 1.375% 1.375%        
Eurobonds Due March 2022 [member]                
Disclosure of detailed information about financial instruments [line items]                
Total committed facilities available £ 211.6   € 250          
Floating basis 3m EURIBOR              
Adjustment to floating basis 0.45% 0.45% 0.45% 0.45%        
Sterling bonds due September 2046 [member]                
Disclosure of detailed information about financial instruments [line items]                
Total committed facilities available | £ £ 400.0              
Bonds interest rate 2.875% 2.875% 2.875% 2.875%        
US revolving credit facilities [member]                
Disclosure of detailed information about financial instruments [line items]                
Total committed facilities available   $ 2,500            
US revolving credit facilities [member] | Weighted average [member]                
Disclosure of detailed information about financial instruments [line items]                
Bonds interest rate 1.11% 1.11% 1.11% 1.11% 0.96% 0.96%    
Borrowings   $ 72       $ 125    
US revolving credit facilities [member] | Weighted average [member]                
Disclosure of detailed information about financial instruments [line items]                
Borrowings averaged equivalent           $ 72    
Australian revolving credit facilities due 2019 (prior to extension) [member]                
Disclosure of detailed information about financial instruments [line items]                
Total committed facilities available       $ 150        
Australian revolving credit facilities due 2020 (after extension) [member]                
Disclosure of detailed information about financial instruments [line items]                
Total committed facilities available       150        
Australian revolving credit facilities due 2021 (after reduction in December 2019) [member]                
Disclosure of detailed information about financial instruments [line items]                
Total committed facilities available       270        
Australian revolving credit facilities due 2021 (before reduction in December 2019) [member]                
Disclosure of detailed information about financial instruments [line items]                
Total committed facilities available       370        
Australian New Zealand revolving credit facilities [member]                
Disclosure of detailed information about financial instruments [line items]                
Borrowings averaged equivalent       $ 310        
US revolving credit facilities march 2024 (prior to extension) [member] | Borrowings and financing [member]                
Disclosure of detailed information about financial instruments [line items]                
Total committed facilities available   2,500            
US commercial paper [member]                
Disclosure of detailed information about financial instruments [line items]                
Borrowings   $ 0            
US commercial paper [member] | Weighted average [member]                
Disclosure of detailed information about financial instruments [line items]                
Bonds interest rate 2.46% 2.46% 2.46% 2.46% 2.28% 2.28%    
Borrowings   $ 41       $ 540    
Euro commercial paper [Member]                
Disclosure of detailed information about financial instruments [line items]                
Borrowings | €     € 0          
Euro commercial paper [Member] | Weighted average [member]                
Disclosure of detailed information about financial instruments [line items]                
Borrowings | £ £ 255.0       £ 0.0