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Pension benefits (Details 3)
Dec. 31, 2017
Dec. 31, 2016
Minimum [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Discount rate 5.78% 5.40%
Rate of increase in compensation level 5.00% 5.00%
Maximum [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Discount rate 10.60% 12.50%
Rate of increase in compensation level 7.00% 8.50%