Product Warranty Reserves (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 24, 2016	Sep. 30, 2017	Sep. 24, 2016
Product Warranties Disclosures [Abstract]
Product warranty reserve at beginning of period	$ 2,145	$ 2,214	$ 2,307	$ 2,159
Warranty liabilities accrued	1,453	1,537	4,365	4,574
Warranty liabilities settled	(1,394)	(1,583)	(4,365)	(5,128)
Changes for pre-existing warranty liabilities	(48)	(45)	(151)	518
Product warranty reserve at end of period	$ 2,156	$ 2,123	$ 2,156	$ 2,123