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Analysis of cash flows - Summary of Analysis of Cash Flows (Parenthetical) (Detail) - Jun. 30, 2018
GBP (£)
USD ($)
EUR (€)
Disclosure Of Cash Flow Statement [Line Items]      
Bond amount | £ £ 8,208,800,000    
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
Partial repayment of USD 300 million bonds [member]      
Disclosure Of Cash Flow Statement [Line Items]      
Bond amount | $   $ 300,000,000  
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