Current Liabilities - Deferred Services Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Apr. 01, 2017	Apr. 02, 2016	Dec. 31, 2016
Balance sheets classification
Deferred service income-short term	$ 5,500		$ 4,600
Deferred service income-long term	895		$ 1,145
Service Contracts
Balance sheets classification
Deferred service income-short term	3,170	$ 4,174
Deferred service income-long term	895	0
Movement in Deferred Revenue [Roll Forward]
Balance, beginning of the period	3,842	4,011
Additions	1,073	1,179
Revenue recognized	(850)	(1,016)
Balance, end of the period	$ 4,065	$ 4,174