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Labor obligations - Actuarial assumptions (Details) - Defined benefit plan - item
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Labor obligations      
Discount rate 8.00% 9.00% 7.75%
Expected rate of salary increase 5.80% 5.80% 4.50%
Average longevity at retirement age for current pensioners (in years) 14 13 14
Inflation 5.00% 4.00% 3.50%