Restructuring and Contract Termination Costs (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 25, 2011
Severance and Retention Costs
Balance, beginning of period	$ 3,374	$ 10,658
Expense	911	1,392
Payments/utilization	(1,233)	(6,065)
Balance, end of period	3,052	5,985
Severance and retention costs	911	1,392
Research Models and Services [Member]
Severance and Retention Costs
Severance and retention costs	0	408
Preclinical Services [Member]
Severance and Retention Costs
Severance and retention costs	911	984
Corporate [Member]
Severance and Retention Costs
Severance and retention costs	0	0
Cost of Sales [Member]
Severance and Retention Costs
Severance and retention costs	0	431
General and Administrative Expense [Member]
Severance and Retention Costs
Severance and retention costs	911	961
Accrued Compensation [Member]
Severance and Retention Costs
Severance and retention costs, current	1,047
Other Long-Term Liabilities [Member]
Severance and Retention Costs
Severance and retention costs, noncurrent	$ 2,005