Segment Information - Sales to External Customers and Gross Profit (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 28, 2014	Sep. 29, 2013	Sep. 28, 2014	Sep. 29, 2013
Segment Reporting Information [Line Items]
Sales to external customers	$ 227,756	$ 218,496	$ 690,976	$ 662,163
Gross profit	108,581	104,745	324,756	314,193
Industry [Member]
Segment Reporting Information [Line Items]
Sales to external customers	107,058	103,627	340,722	306,393
Gross profit	55,101	56,216	175,321	160,517
Science [Member]
Segment Reporting Information [Line Items]
Sales to external customers	120,698	114,869	350,254	355,770
Gross profit	$ 53,480	$ 48,529	$ 149,435	$ 153,676