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Analysis of cash flows - Summary of Analysis of Cash Flows (Parenthetical) (Detail)
Jun. 30, 2019
GBP (£)
Jun. 30, 2019
USD ($)
Jun. 30, 2019
EUR (€)
Dec. 31, 2018
GBP (£)
Jun. 30, 2018
GBP (£)
Disclosure Of Cash Flow Statement [line items]          
Cash and cash equivalents acquired from acquisitions £ 0     £ 11,300,000  
Cash and cash equivalents from overdraft of acquisition         £ 3,900,000
Cash and cash equivalents acquired from disposals 3,000,000     £ 15,100,000 £ 200,000
Bond amount 7,838,200,000        
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 200 million GBP 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      
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    
Partial repayment of Usd 500 million bonds [member]          
Disclosure Of Cash Flow Statement [line items]          
Bond amount | $   $ 500,000,000