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SIGNIFICANT ACCOUNTING POLICIES - Analysis of warranty accruals (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Movement in Standard and Extended Product Warranty, Increase (Decrease) [Roll Forward]    
Balance at beginning of year $ 4,629 $ 3,405
Less: Payments made during the year (2,599) (2,856)
Add: Warranty provision for the current year 3,436 3,527
Changes to warranty provision for prior years (93) 553
Balance at end of year $ 5,373 $ 4,629