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Accrued product warranty liability (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Accrued product warranty liability [Roll Forward]    
Balance, beginning of period $ 267 $ 267
Warranties issued 233 184
Warranty settlements (205) (187)
Balance, end of period 295 $ 264
Accrued product warranty liability, current 197  
Accrued product warranty liability, non-current $ 98  
Maximum [Member]    
Warranty [Abstract]    
Product warranty period 24 months