Product Warranty (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Product Warranty
Product warranty period	1 year
Changes in standard product warranty liability
Balance at January 1 (beginning of year)	$ 5,091	$ 4,502
Warranties issued during the period	954	1,540
Settlements made during the period	(1,475)	(1,334)
Changes in estimate of liability for pre-existing warranties during the period	89	260
Balance at March 31 (end of period)	4,659	4,968
Product warranty classification
Amount classified as current			$ 4,319	$ 4,819	$ 4,583
Amount classified as long-term			340		385
Total warranty liability	$ 5,091	$ 4,502	$ 4,659	$ 5,091	$ 4,968