Product Warranty (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Product Warranty
Product warranty period	1 year
Changes in standard product warranty liability
Balance at January 1 (beginning of year)	$ 2,666	$ 3,555
Warranties issued during the period	3,898	2,286
Settlements made during the period	(1,614)	(3,570)
Changes in estimate of liability for pre-existing warranties during the period	(562)	551
Balance at September 30 (end of period)	4,388	2,822
Product warranty classification
Amount classified as current			$ 4,154	$ 2,426	$ 2,597
Amount classified as long-term			234		225
Total warranty liability	$ 2,666	$ 3,555	$ 4,388	$ 2,666	$ 2,822