Segment Information - Sales to External Customers and Gross Profit (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 28, 2015	Jun. 29, 2014	Jun. 28, 2015	Jun. 29, 2014
Segment Reporting Information [Line Items]
Sales to external customers	$ 224,189	$ 236,955	$ 445,005	$ 463,220
Gross profit	112,105	109,851	217,376	216,176
Industry [Member]
Segment Reporting Information [Line Items]
Sales to external customers	119,580	127,225	231,480	233,664
Gross profit	64,747	64,088	121,357	120,221
Science [Member]
Segment Reporting Information [Line Items]
Sales to external customers	104,609	109,730	213,525	229,556
Gross profit	$ 47,358	$ 45,763	$ 96,019	$ 95,955
Top 10 Customers [Member]
Segment Reporting Information [Line Items]
Concentration Risk, Customer	38.10%	36.90%	33.10%	31.90%