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Post-employment benefits - Assumptions used for defined-benefit obligations in the United States, Germany and the rest of the world (Detail)
Dec. 31, 2020
Dec. 31, 2019
Post-employment benefits [Line Items]    
Actuarial assumption of discount rates 1.50% 2.40%
Actuarial assumption of expected rates of inflation 1.70% 1.90%
Actuarial assumption of expected rates of salary increases 2.50% 2.60%
United States of America [Member]    
Post-employment benefits [Line Items]    
Actuarial assumption of discount rates 2.30% 3.10%
Actuarial assumption of expected rates of inflation 2.00% 2.00%
Actuarial assumption of expected rates of salary increases 0.00% 0.00%
Germany [Member]    
Post-employment benefits [Line Items]    
Actuarial assumption of discount rates 0.60% 0.80%
Actuarial assumption of expected rates of inflation 1.60% 1.80%
Actuarial assumption of expected rates of salary increases 2.50% 2.50%
Other countries [Member]    
Post-employment benefits [Line Items]    
Actuarial assumption of discount rates 2.20% 2.60%
Actuarial assumption of expected rates of inflation 1.70% 1.90%
Actuarial assumption of expected rates of salary increases 2.70% 2.80%