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Supplemental pension plans (Details 3)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Benefit Obligations And Net Periodic Benefit Cost [Abstract]    
Discount rate [1] 8.5% - 10% p.a. 11.1% p.a.
Expected long-term rate of return on the assets 8.5% - 10% p.a. 11.1% p.a.
Increase in salary levels 4.3% p.a. 4.8% p.a.
[1] In 2017, considering an inflation rate of 4.3% p.a. and a real discount rate of 4.0% - 5.5% p.a. (2016 - 4.8% and 6.0% p.a., respectively).