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Debt financing - Additional Information (Detail) - GBP (£)
£ in Millions
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Disclosure of bank overdraft bonds and bank loans [line items]      
Carrying amount of bank loans £ 624.6 £ 993.4 £ 1,343.0
Corporate bond [member]      
Disclosure of bank overdraft bonds and bank loans [line items]      
Fair value of corporate bonds £ 6,035.1 £ 5,816.5 £ 5,868.2