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Labor obligations - Actuarial assumptions (Details) - Defined benefit plan - item
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Labor obligations      
Discount rate 7.40% 8.00% 9.00%
Expected rate of salary increase 5.80% 5.80% 5.80%
Average longevity at retirement age for current pensioners (in years) 12 14 13
Inflation 4.00% 5.00% 4.00%