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Post-employment benefit obligations (Schedule of Movements in Fair Value of Plan Assets) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of net defined benefit liability (asset) [line items]    
Assets, beginning of year $ 15,794  
Assets, end of year 18,112 $ 15,794
Plan Assets [Member]    
Disclosure of net defined benefit liability (asset) [line items]    
Assets, beginning of year 15,794 15,205
Return on assets 678 648
Actuarial gain (loss) 1,555 13
Employer contributions 986 758
Benefits paid (901) (830)
Assets, end of year $ 18,112 $ 15,794