Product Warranty (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Product Warranty
Product warranty period	1 year
Changes in standard product warranty liability
Balance at January 1 (beginning of year)	$ 2,666	$ 3,555	$ 1,526
Warranties issued during the period	5,671	3,125	4,305
Settlements made during the period	(2,603)	(4,249)	(2,672)
Changes in estimate of liability for pre-existing warranties during the period	(1,232)	235	396
Balance at December 31 (end of period)	4,502	2,666	3,555
Product warranty classification
Amount classified as current				$ 4,112	$ 2,426	$ 3,363
Amount classified as long-term				390	240	192
Total warranty liability	$ 2,666	$ 3,555	$ 1,526	$ 4,502	$ 2,666	$ 3,555