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Ordinary dividends - Summary of Ordinary Dividends (Detail) - GBP (£)
£ / shares in Units, £ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of Dividends [line items]      
Amounts recognised as distributions to equity holders, per share £ 0.6000 £ 0.6000 £ 0.5975
Amounts recognised as distributions to equity holders £ 750.5 £ 747.4 £ 751.5
Proposed final dividend, per share £ 0.3730    
Previous period final amount [member]      
Disclosure of Dividends [line items]      
Amounts recognised as distributions to equity holders, per share £ 0.3730 £ 0.3730 £ 0.3705
Amounts recognised as distributions to equity holders £ 466.4 £ 464.6 £ 467.2
Interim period amount [member]      
Disclosure of Dividends [line items]      
Amounts recognised as distributions to equity holders, per share £ 0.2270 £ 0.2270 £ 0.2270
Amounts recognised as distributions to equity holders £ 284.1 £ 282.8 £ 284.3