Goodwill and Other Intangible Assets (Gross Carrying Amount and Accumulated Amortization of Goodwill) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 27, 2014	Dec. 28, 2013	Dec. 29, 2012
Goodwill [Line Items]
Goodwill, Gross	$ 1,326,077	$ 1,235,701	$ 1,213,609
Goodwill	321,077	230,701	208,609
Research Models and Services
Goodwill [Line Items]
Goodwill, Gross	59,196	83,551	63,139
Goodwill, Transfers	(23,172)
Goodwill, Acquired During Period	0	19,647
Goodwill, Other Changes	(1,183)	765
Discovery and Safety Assessment
Goodwill [Line Items]
Goodwill, Gross	1,234,302	1,152,150	1,150,470
Goodwill, Impaired, Accumulated Impairment Loss	(1,005,000)	(1,005,000)	(1,005,000)
Goodwill, Transfers	(9,196)
Goodwill, Acquired During Period	102,171	0
Goodwill, Other Changes	(10,823)	1,680
Manufacturing Support [Member]
Goodwill [Line Items]
Goodwill, Gross	32,579	0	0
Goodwill, Transfers	32,368
Goodwill, Acquired During Period		0
Goodwill, Other Changes	$ 211	$ 0