Business Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2012	Mar. 31, 2012 segments	Jun. 25, 2011	Jun. 30, 2012	Jun. 25, 2011
Segment Reporting [Abstract]
Number of reportable segments		2
Business segment information
Net sales	$ 284,723		$ 288,263	$ 570,704	$ 574,106
Operating income	49,274		53,314	93,014	95,565
Depreciation and amortization				40,067	43,081
Capital expenditures				23,553	13,450
Unallocated corporate overhead
Stock-based compensation expense	5,321		5,430	10,586	11,348
Depreciation expense				31,161	31,904
Tax Exempt Gain on Settlement of Life Insurance	0		(7,710)	0	(7,710)
Research Models [Member]
Business segment information
Net sales	87,119		90,190	181,140	183,590
Research Model Services [Member]
Business segment information
Net sales	53,975		55,422	110,306	107,397
Other Products [Member]
Business segment information
Net sales	32,517		32,551	65,317	60,547
Total Segments [Member]
Business segment information
Operating income	66,351		63,566	129,992	124,614
Research Models and Services [Member]
Business segment information
Net sales	173,611		178,163	356,763	351,534
Gross margin	76,266		78,307	158,462	152,146
Operating income	55,542		55,691	115,009	107,433
Depreciation and amortization	9,085		9,318	18,027	18,587
Capital expenditures	7,569		4,010	20,469	8,413
Preclinical Services [Member]
Business segment information
Net sales	111,112		110,100	213,941	222,572
Gross margin	27,319		28,013	49,335	56,812
Operating income	10,809		7,875	14,983	17,181
Depreciation and amortization	10,980		12,498	22,040	24,494
Capital expenditures	1,872		2,650	3,084	5,037
Unallocated Corporate Overhead [Member]
Business segment information
Operating income	(17,077)		(10,252)	(36,978)	(29,049)
Unallocated corporate overhead
Stock-based compensation expense	2,900		2,528	5,685	5,514
U.S. retirement plans	1,014		1,055	2,386	2,112
Audit, tax and related expenses	637		505	1,291	1,260
Salary and bonus	4,866		4,642	9,789	9,335
Global IT	3,366		2,794	6,216	5,787
Employee health, long-term disability and fringe benefit expense	(1,140)		675	853	2,314
Consulting and professional services	778		2,201	2,520	3,532
Depreciation expense	1,570		1,593	3,139	3,174
Other general unallocated corporate expenses	3,086		1,969	5,099	3,731
Total unallocated corporate overhead costs	$ 17,077		$ 10,252	$ 36,978	$ 29,049