Business Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 28, 2013 segments	Sep. 29, 2012	Sep. 28, 2013	Sep. 29, 2012
Segment Reporting [Abstract]
Number of reportable segments	2
Business segment information
Net sales	$ 292,129	$ 278,686	$ 876,300	$ 849,390
Operating income	40,843	37,682	126,794	130,696
Depreciation and amortization			67,336	60,617
Capital expenditures			25,319	33,795
Unallocated corporate overhead
Stock-based compensation expense	6,047	5,241	18,232	15,828
Depreciation expense			54,444	47,181
Research Models [Member]
Business segment information
Net sales	92,969	90,877	294,993	293,575
Research Model Services [Member]
Business segment information
Net sales	52,105	53,400	156,661	163,247
Other Products [Member]
Business segment information
Net sales	28,331	22,207	83,213	66,425
Total Segments [Member]
Business segment information
Operating income	58,896	54,364	182,824	184,356
Research Models and Services [Member]
Business segment information
Net sales	173,405	166,484	534,867	523,247
Gross margin	65,710	65,902	221,916	224,364
Operating income	40,260	43,389	145,193	158,398
Depreciation and amortization	16,876	9,670	37,378	27,697
Capital expenditures	6,110	7,423	16,464	27,892
Preclinical Services [Member]
Business segment information
Net sales	118,724	112,202	341,433	326,143
Gross margin	34,216	27,358	84,791	76,693
Operating income	18,636	10,975	37,631	25,958
Depreciation and amortization	10,039	10,880	29,957	32,920
Capital expenditures	2,986	2,819	8,855	5,903
Unallocated Corporate Overhead [Member]
Business segment information
Operating income	(18,053)	(16,682)	(56,030)	(53,660)
Unallocated corporate overhead
Stock-based compensation expense	3,260	2,827	9,927	8,512
U.S. retirement plans	1,275	1,276	3,617	3,662
Audit, tax and related expenses	811	842	3,135	2,133
Salary and bonus	5,831	4,813	16,057	14,602
Global IT	3,002	3,285	8,448	9,501
Employee health, long-term disability and fringe benefit expense	(1,470)	(2,248)	(898)	(1,395)
Consulting and professional services	1,439	1,061	3,443	3,581
Depreciation expense	1,570	1,554	4,712	4,693
Other general unallocated corporate expenses	2,335	3,272	7,589	8,371
Total unallocated corporate overhead costs	$ 18,053	$ 16,682	$ 56,030	$ 53,660