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Analysis of cash flows - Summary of Analysis of Cash Flows (Parenthetical) (Detail) - Jun. 30, 2020
GBP (£)
EUR (€)
USD ($)
Disclosure Of Cash Flow Statement [line items]      
Bond amount | £ £ 7,435,500,000    
Repayment of EURO 250 million bonds [member]      
Disclosure Of Cash Flow Statement [line items]      
Bond amount | €   € 250,000,000  
Repayment of GBP 200 million bonds [member]      
Disclosure Of Cash Flow Statement [line items]      
Bond amount | £ 200,000,000    
Partial repayment of USD 450 million bonds [member]      
Disclosure Of Cash Flow Statement [line items]      
Bond amount | $     $ 450,000,000
Partial repayment of USD 272 million bonds [member]      
Disclosure Of Cash Flow Statement [line items]      
Bond amount | $     272,000,000
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
Proceeds from issue of EURO 750 million bonds [member]      
Disclosure Of Cash Flow Statement [line items]      
Bond amount | €   € 750,000,000  
Proceeds from issue of GBP 250 million bonds [Member]      
Disclosure Of Cash Flow Statement [line items]      
Bond amount | £ £ 250,000,000