Accrued product warranty liability (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Accrued product warranty liability [Roll Forward]
Balance, beginning of period	$ 267	$ 277
Warranties issued	105	131
Warranty settlements	(123)	(136)
Balance, end of period	249	$ 272
Accrued product warranty liability, current	168
Accrued product warranty liability, long-term	$ 81
Maximum [Member]
Warranty [Abstract]
Product warranty period	24 months