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Post-employment benefit obligations (Narrative) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of defined benefit plans [line items]    
Increase (decrease) in defined benefit obligation that would have been caused by a 1% decrease in discount rate $ 4,400 $ 3,800
Increase (decrease) in defined benefit obligation that would have been caused by a 1% increase in discount rate $ 100 $ 100
Expected percentage return on plan assets 4.30% 4.20%
Expected contributions to pension benefit plans $ 800  
Actuarial gain (loss) on post-employment benefit obligations $ (394) $ 1,607
Expected average service life of active plan participants 9 years  
Bottom of range [member]    
Disclosure of defined benefit plans [line items]    
Increase (decrease) in defined benefit obligation $ 26,700 26,700
Top of range [member]    
Disclosure of defined benefit plans [line items]    
Increase (decrease) in defined benefit obligation $ 31,100 $ 26,800