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Significant accounting policies (income taxes, derivative financial instruments, revenue recognition and employee share-based compensation) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Income taxes    
Measurement of tax benefit, minimum percentage tax benefit must be likely to be realized 50.00%  
Derivative financial instruments    
Net fair value of derivatives $ 0  
Revenue recognition    
Costs and earnings in excess of billings 6,729 $ 7,471
Billings in excess of costs and earnings $ 1,266 $ 569
Employee stock purchase plan [Member]    
Share-based compensation plans [Line Items]    
Employee stock purchase plan discount 15.00%  
Purchase period 6 months