Commitments and Contingencies Warranty Liabilities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017
Warranty Liabilities [Roll Forward]
Balance at beginning of year	$ 9,320	$ 8,633
Additions charged to costs and expenses for current-year sales	6,106	4,243
Deductions from reserves	(5,790)	(3,665)
Changes in liability for pre-existing warranties during the current year, including expirations	132	(55)
Balance at end of period	$ 9,768	$ 9,156