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Provision for post-employment benefits - Surplus/(Deficit) in Plans by Region (Detail) - GBP (£)
£ in Millions
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of defined benefit plans [line items]      
Surplus/(deficit) in plans £ (159.0) £ (183.4) £ (205.4)
United Kingdom [member]      
Disclosure of defined benefit plans [line items]      
Surplus/(deficit) in plans 0.3 33.7 31.5
North America [member]      
Disclosure of defined benefit plans [line items]      
Surplus/(deficit) in plans (45.2) (68.7) (89.2)
Western Continental Europe [member]      
Disclosure of defined benefit plans [line items]      
Surplus/(deficit) in plans (79.4) (104.6) (107.7)
Asia Pacific, Latin America, Africa & Middle East and Central & Eastern Europe [member]      
Disclosure of defined benefit plans [line items]      
Surplus/(deficit) in plans £ (34.7) £ (43.8) £ (40.0)