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Post-employment benefits - Assumptions used for defined-benefit obligations (Detail)
Dec. 31, 2018
Dec. 31, 2017
Post-employment benefits [Abstract]    
Actuarial assumption of discount rates 3.20% 2.80%
Actuarial assumption of expected rates of inflation 2.10% 2.10%
Actuarial assumption of expected rates of salary increases 2.40% 2.40%