Accrued product warranty liability (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accrued product warranty liability [Roll Forward]
Balance, beginning of period	$ 267	$ 267
Warranties issued	80	55
Warranty settlements	(69)	(65)
Balance, end of period	278	$ 257
Accrued product warranty liability, current	196
Accrued product warranty liability, long-term	$ 82
Maximum [Member]
Warranty [Abstract]
Product warranty period	36 months