Revenue Recognition- Additional Information (Detail) - USD ($)	3 Months Ended	6 Months Ended
Jun. 28, 2019	Jun. 29, 2018	Jun. 28, 2019	Jun. 29, 2018
Revenues	$ 106,506,000	$ 95,621,000	$ 205,537,000	$ 192,078,000
Concentration Risk, Percentage	10.00%	10.00%	10.00%	10.00%
Deferred Revenue, Revenue Recognized	$ 1,876,000		$ 4,481,000
Subcontractor Fees Not Included In Revenues	6,200,000	$ 5,800,000	10,400,000	$ 11,400,000
Engineering and Other Scientific [Member] \| Time And Materials Contracts [Member]
Revenues	71,761,000	62,347,000	137,140,000	126,948,000
Engineering and Other Scientific [Member] \| Fixed Price Contracts [Member]
Revenues	15,059,000	14,733,000	29,934,000	27,179,000
Environmental and Health [Member] \| Time And Materials Contracts [Member]
Revenues	18,792,000	17,618,000	36,810,000	36,188,000
Environmental and Health [Member] \| Fixed Price Contracts [Member]
Revenues	894,000	923,000	1,653,000	1,763,000
Sales Revenue, Net [Member] \| Time And Materials Contracts [Member]
Revenues	$ 90,553,000	$ 79,965,000	$ 173,950,000	$ 163,136,000
Sales Revenue, Net [Member] \| Time And Materials Contracts [Member] \| Revenue from Rights Concentration Risk [Member]
Concentration Risk, Percentage	85.00%	84.00%	85.00%	85.00%
Sales Revenue, Net [Member] \| Fixed Price Contracts [Member]
Revenues	$ 15,953,000	$ 15,656,000	$ 31,587,000	$ 28,942,000
Sales Revenue, Net [Member] \| Fixed Price Contracts [Member] \| Revenue from Rights Concentration Risk [Member]
Concentration Risk, Percentage	15.00%	16.00%	15.00%	15.00%

The total amount of subcontractor fees not included in revenues.

No definition available.

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- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
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- Definition

Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
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-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130549-203045

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- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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