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Provision for post-employment benefits - Surplus/(Deficit) in Plans by Region (Detail) - GBP (£)
£ in Millions
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of defined benefit plans [line items]      
Surplus/(deficit) in plans £ (183.4) £ (205.4) £ (275.6)
United Kingdom [member]      
Disclosure of defined benefit plans [line items]      
Surplus/(deficit) in plans 33.7 31.5 20.0
North America [member]      
Disclosure of defined benefit plans [line items]      
Surplus/(deficit) in plans (68.7) (89.2) (133.8)
Western Continental Europe [member]      
Disclosure of defined benefit plans [line items]      
Surplus/(deficit) in plans (104.6) (107.7) (116.9)
Asia Pacific, Latin America, Africa & Middle East and Central & Eastern Europe [member]      
Disclosure of defined benefit plans [line items]      
Surplus/(deficit) in plans £ (43.8) £ (40.0) £ (44.9)