Geographic and Group Information (Geographic Revenue before Reimbursements) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 27, 2020	Mar. 29, 2019
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue before reimbursements	$ 69,533	$ 67,155
Revenue Before Reimbursements [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue before reimbursements	65,186	62,370
Revenue Before Reimbursements [Member] \| North America [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue before reimbursements	57,054	52,537
Revenue Before Reimbursements [Member] \| International (Primarily European Countries) [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue before reimbursements	$ 8,132	$ 9,833

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045

Name:	us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration