Segment Information - Sales to External Customers and Gross Profit (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jul. 01, 2012	Jun. 30, 2013	Jul. 01, 2012
Segment Reporting Information [Line Items]
Sales to external customers	$ 222,478	$ 221,452	$ 443,667	$ 439,007
Gross profit	106,897	104,429	209,448	202,540
Industry [Member]
Segment Reporting Information [Line Items]
Sales to external customers	103,702	117,353	202,766	229,039
Gross profit	53,788	59,920	104,302	116,508
Science [Member]
Segment Reporting Information [Line Items]
Sales to external customers	118,776	104,099	240,901	209,968
Gross profit	$ 53,109	$ 44,509	$ 105,146	$ 86,032