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Authorised and issued share capital - Range of Exercise Prices of Options (Detail) - 12 months ended Dec. 31, 2019
USD ($)
£ / shares
Ordinary shares [member]    
Disclosure of range of exercise prices of outstanding share options [line items]    
Weighted average contractual life (Months) 90 months  
American depository receipts [member]    
Disclosure of range of exercise prices of outstanding share options [line items]    
Weighted average contractual life (Months) 89 months  
Bottom of range [member] | Ordinary shares [member]    
Disclosure of range of exercise prices of outstanding share options [line items]    
Exercise price | £ / shares   £ 6.268
Bottom of range [member] | American depository receipts [member]    
Disclosure of range of exercise prices of outstanding share options [line items]    
Exercise price | $ $ 49.230  
Top of range [member] | Ordinary shares [member]    
Disclosure of range of exercise prices of outstanding share options [line items]    
Exercise price | £ / shares   17.055
Top of range [member] | American depository receipts [member]    
Disclosure of range of exercise prices of outstanding share options [line items]    
Exercise price | $ 115.940  
Weighted average [member] | Ordinary shares [member]    
Disclosure of range of exercise prices of outstanding share options [line items]    
Exercise price | £ / shares   £ 12.171
Weighted average [member] | American depository receipts [member]    
Disclosure of range of exercise prices of outstanding share options [line items]    
Exercise price | $ $ 83.488