Segment Information - Revenue and Other Financial Information by Reportable Segment (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 26, 2015 USD ($)	Sep. 27, 2014 USD ($)	Sep. 26, 2015 USD ($) segments	Sep. 27, 2014 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segments			3
Segment Reporting Information [Line Items]
Revenue	$ 349,465	$ 327,567	$ 1,009,452	$ 968,114
Operating income	55,440	46,172	154,180	136,903
Depreciation and amortization	23,814	26,068	69,330	70,435
Capital expenditures	10,452	9,402	35,008	29,907
Operating segments
Segment Reporting Information [Line Items]
Operating income	83,109	66,605	234,309	204,665
Depreciation and amortization	21,967	24,130	63,806	64,769
Capital expenditures	9,438	9,009	31,342	28,302
Research Models and Services | Operating segments
Segment Reporting Information [Line Items]
Revenue	118,451	124,021	358,506	389,636
Gross margin	47,841	45,295	143,352	149,392
Operating income	31,494	28,056	93,800	97,734
Depreciation and amortization	5,319	7,277	16,712	20,277
Capital expenditures	3,022	4,110	12,111	11,528
Discovery and Safety Assessment | Operating segments
Segment Reporting Information [Line Items]
Revenue	158,272	140,862	451,659	388,614
Gross margin	53,721	39,968	146,660	105,084
Operating income	33,191	19,329	84,856	48,840
Depreciation and amortization	11,509	13,340	35,060	33,867
Capital expenditures	4,277	3,436	13,756	11,330
Manufacturing Support | Operating segments
Segment Reporting Information [Line Items]
Revenue	72,742	62,684	199,287	189,864
Gross margin	36,513	33,005	100,506	98,239
Operating income	18,424	19,220	55,653	58,091
Depreciation and amortization	5,139	3,513	12,034	10,625
Capital expenditures	$ 2,139	$ 1,463	$ 5,475	$ 5,444