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Analysis of cash flows - Summary of Analysis of Cash Flows (Parenthetical) (Detail) - Dec. 31, 2019
GBP (£)
USD ($)
EUR (€)
Disclosure Of Cash Flow Statement [line items]      
Bond amount | £ £ 6,309,800,000    
Repayment of EURO 600 million bonds [member]      
Disclosure Of Cash Flow Statement [line items]      
Bond amount     € 600,000,000
Repayment of USD 812 million bonds [member]      
Disclosure Of Cash Flow Statement [line items]      
Bond amount | $   $ 812,000,000  
Partial repayment of USD 272 million bonds [member]      
Disclosure Of Cash Flow Statement [line items]      
Bond amount | $   272,000,000  
Partial repayment of USD 450 million bonds [member]      
Disclosure Of Cash Flow Statement [line items]      
Bond amount | $   $ 450,000,000  
Repayment of GBP 200 million bonds [member]      
Disclosure Of Cash Flow Statement [line items]      
Bond amount | £ 200,000,000    
Proceeds from issue of EURO 250 million bonds [member]      
Disclosure Of Cash Flow Statement [line items]      
Bond amount     250,000,000
Proceeds from issue of EURO 500 million bonds [member]      
Disclosure Of Cash Flow Statement [line items]      
Bond amount     500,000,000
Repayment of EURO 252 million bonds [member]      
Disclosure Of Cash Flow Statement [line items]      
Bond amount     € 252,000,000
Repayment of GBP 400 million bonds [member]      
Disclosure Of Cash Flow Statement [line items]      
Bond amount | £ £ 400,000,000