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Share-based payments - Movement on Ordinary Shares Granted for Significant Restricted Stock Plans (Detail)
pure in Millions
 12 Months Ended
Dec. 31, 2018
GBP (£)
Executive Performance Share Plan [member]  
Disclosure of movement on ordinary shares granted for significant restricted stock plans [line items]  
Weighted average fair value, Non-vested beginning balance £ 13.68
Weighted average fair value, Granted 8.14
Weighted average fair value, Lapsed 15.00
Weighted average fair value, Vested 11.22
Weighted average fair value, Non-vested ending balance £ 13.63
Number of shares, Non-vested beginning balance 9.1
Number of shares, Granted 0.7
Number of shares, Lapsed (1.1)
Number of shares, Vested (2.0)
Number of shares, Non-vested ending balance 6.7
Performance Share Awards [member]  
Disclosure of movement on ordinary shares granted for significant restricted stock plans [line items]  
Weighted average fair value, Non-vested beginning balance £ 16.59
Weighted average fair value, Granted 12.02
Weighted average fair value, Lapsed 15.22
Weighted average fair value, Vested 15.17
Weighted average fair value, Non-vested ending balance £ 14.37
Number of shares, Non-vested beginning balance 1.9
Number of shares, Granted 1.4
Number of shares, Lapsed (0.1)
Number of shares, Vested (0.9)
Number of shares, Non-vested ending balance 2.3
Leaders, Partners and High Potential Group [member]  
Disclosure of movement on ordinary shares granted for significant restricted stock plans [line items]  
Weighted average fair value, Non-vested beginning balance £ 15.02
Weighted average fair value, Granted 8.06
Weighted average fair value, Lapsed 14.72
Weighted average fair value, Vested 14.26
Weighted average fair value, Non-vested ending balance £ 11.54
Number of shares, Non-vested beginning balance 6.8
Number of shares, Granted 4.8
Number of shares, Lapsed (0.7)
Number of shares, Vested (1.8)
Number of shares, Non-vested ending balance 9.1