Geographic and Group Information (Geographic Revenue before Reimbursements) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 29, 2018	Jun. 30, 2017	Jun. 29, 2018	Jun. 30, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue before reimbursements	$ 75,649	$ 73,570	$ 148,382	$ 144,999
Revenue Before Reimbursements [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue before reimbursements	69,614	67,726	137,089	132,795
Revenue Before Reimbursements [Member] | North America [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue before reimbursements	57,245	52,607	110,771	105,644
Revenue Before Reimbursements [Member] | International (Primarily European Countries) [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue before reimbursements	$ 12,369	$ 15,119	$ 26,318	$ 27,151