Product Warranty (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 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	1,540	1,166
Settlements made during the period	(1,334)	(454)
Changes in estimate of liability for pre-existing warranties during the period	260	(254)
Balance at March 31 (end of period)	4,968	3,124
Product warranty classification
Amount classified as current			$ 4,583	$ 4,112	$ 2,748
Amount classified as long-term			385		376
Total warranty liability	$ 4,502	$ 2,666	$ 4,968	$ 4,502	$ 3,124