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Share-based payments - Movement on Ordinary Shares Granted for Significant Restricted Stock Plans (Detail)
Unit_pure in Millions
 12 Months Ended
Dec. 31, 2019
£ / shares
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 £ 13.63
Weighted average fair value, Granted 9.89
Weighted average fair value, Lapsed 13.34
Weighted average fair value, Vested 12.65
Weighted average fair value, Non-vested ending balance £ 11.98
Number of shares, Non-vested beginning balance 6.7
Number of shares, Granted 4.2
Number of shares, Lapsed (1.3)
Number of shares, Vested (0.8)
Number of shares, Non-vested ending balance 8.8
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 £ 14.37
Weighted average fair value, Granted 9.26
Weighted average fair value, Lapsed 12.10
Weighted average fair value, Vested 15.72
Weighted average fair value, Non-vested ending balance £ 10.81
Number of shares, Non-vested beginning balance 2.3
Number of shares, Granted 1.7
Number of shares, Lapsed (0.4)
Number of shares, Vested (1.0)
Number of shares, Non-vested ending balance 2.6
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 £ 11.54
Weighted average fair value, Granted 9.09
Weighted average fair value, Lapsed 10.76
Weighted average fair value, Vested 15.51
Weighted average fair value, Non-vested ending balance £ 9.74
Number of shares, Non-vested beginning balance 9.1
Number of shares, Granted 4.1
Number of shares, Lapsed (1.9)
Number of shares, Vested (2.0)
Number of shares, Non-vested ending balance 9.3