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Provision for post-employment benefits - Weighted Average Assumptions Used For Actuarial Valuations (Detail)
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
United Kingdom [member]        
Disclosure of defined benefit plans [line items]        
Discount rate 2.00% 2.80% 2.40% 2.50%
Rate of increase in salaries       3.50%
Rate of increase in pensions in payment 4.40% 4.30% 4.10% 4.10%
Inflation 2.60% 2.80% 2.70% 2.80%
North America [member]        
Disclosure of defined benefit plans [line items]        
Discount rate 3.00% 4.10% 3.50% 3.80%
Rate of increase in salaries 3.00% 3.00% 3.10% 3.10%
Inflation     4.00% 4.00%
Western Continental Europe [member]        
Disclosure of defined benefit plans [line items]        
Discount rate 1.20% 2.00% 1.90% 1.70%
Rate of increase in salaries 2.20% 2.30% 1.90% 2.00%
Rate of increase in pensions in payment 1.80% 1.20% 1.20% 1.30%
Inflation 1.70% 1.70% 1.70% 1.70%
Asia Pacific, Latin America, Africa & Middle East and Central & Eastern Europe [member]        
Disclosure of defined benefit plans [line items]        
Discount rate 4.60% 5.00% 4.20% 4.20%
Rate of increase in salaries 6.10% 5.80% 5.50% 5.90%
Inflation 3.70% 3.60% 4.00% 4.00%