Geographic and Group Information (Geographic Revenue before Reimbursements) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 28, 2018	Sep. 29, 2017	Sep. 28, 2018	Sep. 29, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue before reimbursements	$ 73,840	$ 71,462	$ 222,222	$ 216,461
Revenue Before Reimbursements [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue before reimbursements	68,243	65,947	205,332	198,742
Revenue Before Reimbursements [Member] | North America [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue before reimbursements	55,321	53,142	166,092	158,786
Revenue Before Reimbursements [Member] | International (Primarily European Countries) [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue before reimbursements	$ 12,922	$ 12,805	$ 39,240	$ 39,956