Revenue Recognition- Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 28, 2018	Sep. 29, 2017	Sep. 28, 2018	Sep. 29, 2017
Revenues	$ 95,302,000	$ 87,555,000	$ 287,380,000	$ 259,517,000
Deferred Revenue, Revenue Recognized	4,163,000		5,332,000
Subcontractor Fees Not Included In Revenues	4,700,000		21,900,000
Engineering and Other Scientific [Member] | Time And Materials Contracts [Member]
Revenues	59,582,000		186,486,000
Engineering and Other Scientific [Member] | Fixed Price Contracts [Member]
Revenues	18,315,000		45,494,000
Environmental and Health [Member] | Time And Materials Contracts [Member]
Revenues	16,755,000		52,987,000
Environmental and Health [Member] | Fixed Price Contracts [Member]
Revenues	650,000		2,413,000
Sales Revenue, Net [Member] | Time And Materials Contracts [Member]
Revenues	$ 76,337,000		$ 239,473,000
Concentration Risk, Percentage	80.00%		83.00%
Sales Revenue, Net [Member] | Fixed Price Contracts [Member]
Revenues	$ 18,965,000		$ 47,907,000
Concentration Risk, Percentage	20.00%		17.00%