Product Warranty (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 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	3,124	2,577
Settlements made during the period	(2,892)	(1,090)
Changes in estimate of liability for pre-existing warranties during the period	411	(281)
Balance at June 30 (end of period)	5,145	3,872
Product warranty classification
Amount classified as current			$ 4,698	$ 4,112	$ 3,537
Amount classified as long-term			447		335
Total warranty liability	$ 4,502	$ 2,666	$ 5,145	$ 4,502	$ 3,872