Business Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 28, 2014	Jun. 29, 2013	Jun. 28, 2014	Jun. 29, 2013
Business segment information
Revenue	$ 341,179	$ 292,933	$ 640,547	$ 584,171
Operating income	51,025	43,188	90,731	85,951
Depreciation and amortization			44,352	40,420
Capital expenditures			20,505	16,223
Unallocated corporate overhead
Stock-based compensation expense	8,222	6,281	14,881	12,184
Depreciation expense	17,449	15,947	33,159	31,708
Research Models [Member]
Business segment information
Revenue	87,814	85,959	174,796	176,216
Research Model Services [Member]
Business segment information
Revenue	45,306	43,800	90,819	88,416
Research Models and Services [Member]
Business segment information
Revenue	133,120	129,759	265,615	264,632
Other Products [Member]
Business segment information
Revenue	31,866	28,013	61,135	55,375
Manufacturing Support [Member]
Business segment information
Revenue	65,445	55,684	127,180	110,258
Total Segments [Member]
Business segment information
Operating income	72,487	60,565	138,060	123,928
Research Models and Services [Member]
Business segment information
Revenue	133,120	129,759	265,615	264,632
Gross margin	52,450	49,020	104,097	104,476
Operating income	34,234	33,296	69,678	73,773
Depreciation and amortization	7,165	7,192	14,364	14,094
Capital expenditures	2,478	3,991	7,711	6,209
Preclinical Services [Member]
Business segment information
Revenue	142,614	107,490	247,752	209,281
Gross margin	39,457	25,534	65,116	48,051
Operating income	17,798	11,261	29,511	19,704
Depreciation and amortization	13,241	9,369	22,012	18,783
Capital expenditures	4,340	3,064	8,051	4,856
Manufacturing Support [Member]
Business segment information
Revenue	65,445	55,684	127,180	110,258
Gross margin	33,727	28,016	65,234	54,254
Operating income	20,455	16,008	38,871	30,451
Depreciation and amortization	3,894	3,849	7,975	7,543
Capital expenditures	2,497	2,740	4,743	5,158
Unallocated Corporate Overhead [Member]
Business segment information
Operating income	(21,462)	(17,377)	(47,329)	(37,977)
Unallocated corporate overhead
Stock-based compensation expense	4,905	3,470	8,607	6,667
U.S. retirement plans	809	1,042	1,453	2,342
Audit, tax and related expenses	1,226	1,089	3,131	2,324
Salary and bonus	6,919	5,471	13,058	10,226
Global IT	3,152	2,860	6,021	5,446
Employee health, long-term disability and fringe benefit expense	(1,685)	(1,656)	634	572
Consulting and professional services	893	1,315	2,075	2,003
Depreciation expense	1,599	1,572	3,199	3,142
Acquisition Costs, Period Cost	1,371	194	4,676	680
Other general unallocated corporate expenses	2,273	2,020	4,475	4,575
Total unallocated corporate overhead costs	21,462	17,377	47,329	37,977
Endotoxoin and Microbial Detection [Member]
Business segment information
Revenue	$ 33,579	$ 27,671	$ 66,045	$ 54,883