Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 21,714	$ 14,223	$ 15,087	$ 10,336
Components of other comprehensive income/(loss), net of tax:
Foreign currency translation adjustments	(747)	881	(1,579)	1,203
Amortization of defined benefit pension items:
Net actuarial loss	135	147	269	294
Prior service cost	12	10	24	20
Defined benefit pension plan adjustments	147	157	293	314
Other comprehensive (loss)/income, net of tax	(600)	1,038	(1,286)	1,517
Comprehensive income	$ 21,114	$ 15,261	$ 13,801	$ 11,853