Product Warranty (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Product Warranty
Product warranty period	1 year
Changes in standard product warranty liability
Balance at January 1 (beginning of year)	$ 4,502	$ 2,666	$ 3,555
Warranties issued during the period	5,421	5,671	3,125
Settlements made during the period	(5,903)	(2,603)	(4,249)
Changes in estimate of liability for pre-existing warranties during the period	1,071	(1,232)	235
Balance at December 31 (end of period)	5,091	4,502	2,666
Product warranty classification
Amount classified as current				$ 4,819	$ 4,112	$ 2,426
Amount classified as long-term				272	390	240
Total warranty liability	$ 4,502	$ 2,666	$ 3,555	$ 5,091	$ 4,502	$ 2,666