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Employee benefits (Details Textual) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Employee benefits [Line Items]      
Actuarial assumption of discount rates 3.68% 4.64% 5.114%
Key management personnel compensation $ 22,598 $ 27,907 $ 27,367
Number of shares effected 83,609    
Weighted average share price $ 54    
Executive Compensation Plan [Member]      
Employee benefits [Line Items]      
Key management personnel compensation $ 806 $ 3,754  
Number of shares effected 427,652 476,302  
Men [Member]      
Employee benefits [Line Items]      
Actuarial assumption of retirement age 65 years 65 years 65 years
Men [Member] | Top of range [member]      
Employee benefits [Line Items]      
Actuarial assumption of retirement age 60 years    
Women [Member]      
Employee benefits [Line Items]      
Actuarial assumption of retirement age 60 years 60 years 60 years
Women [Member] | Top of range [member]      
Employee benefits [Line Items]      
Actuarial assumption of retirement age 65 years