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Retirement Benefit and Other Post-retirement Obligations - Summary of Changes in Value of US PRMB (Detail) - GBP (£)
£ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of defined benefit plans [line items]      
Opening defined benefit obligation £ (67)    
Interest on plan liabilities 76 £ 86 £ 99
Closing defined benefit obligation (49) (67)  
PRMB [member]      
Disclosure of defined benefit plans [line items]      
Interest on plan liabilities 2 2 3
Present value of defined benefit obligation [member]      
Disclosure of defined benefit plans [line items]      
Exchange differences (3) 13  
Current service cost 9 9  
Curtailments (8)    
Interest on plan liabilities (74) (84)  
Actuarial gains/(losses) - experience (51) 132  
Actuarial gains/(losses) - demographic (12) 134  
Actuarial gains/(losses) - financial 137 39  
Benefits paid 151 206  
Present value of defined benefit obligation [member] | PRMB [member]      
Disclosure of defined benefit plans [line items]      
Opening defined benefit obligation (67) (77)  
Exchange differences (2) 5  
Current service cost 1 1  
Curtailments 11    
Interest on plan liabilities (2) (2)  
Actuarial gains/(losses) - experience 4 1  
Actuarial gains/(losses) - demographic   1  
Actuarial gains/(losses) - financial 2 (2)  
Benefits paid 4 6  
Closing defined benefit obligation £ (49) £ (67) £ (77)