Product Warranty (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Product Warranty
Product warranty period	1 year
Changes in standard product warranty liability
Balance at January 1 (beginning of year)	$ 4,502	$ 2,666
Warranties issued during the period	4,294	3,898
Settlements made during the period	(4,566)	(1,614)
Changes in estimate of liability for pre-existing warranties during the period	650	(562)
Balance at September 30 (end of period)	4,880	4,388
Product warranty classification
Amount classified as current			$ 4,545	$ 4,112	$ 4,154
Amount classified as long-term			335		234
Total warranty liability	$ 4,502	$ 2,666	$ 4,880	$ 4,502	$ 4,388