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Ordinary dividends - Additional Information (Detail) - £ / shares
 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Interim period amount [member]    
Disclosure of Dividends [line items]    
Amounts recognized as distributions to equity holders, per share £ 0.227 £ 0.227
Dividends payable date Nov. 05, 2018  
Previous period final amount [member]    
Disclosure of Dividends [line items]    
Amounts recognized as distributions to equity holders, per share £ 0.373  
Dividends paid date Jul. 09, 2018