Revenue Recognition- Summary of Revenues (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 27, 2019	Sep. 28, 2018	Sep. 27, 2019	Sep. 28, 2018
Disaggregation Of Revenue [Line Items]
Revenues	$ 101,548	$ 95,302	$ 307,085	$ 287,380
Concentration Risk, Percentage	10.00%	10.00%	10.00%	10.00%
Time And Materials Contracts [Member] | Revenue from Rights Concentration Risk [Member]
Disaggregation Of Revenue [Line Items]
Concentration Risk, Percentage	85.30%	80.10%	84.90%	83.30%
Fixed Price Contracts [Member] | Revenue from Rights Concentration Risk [Member]
Disaggregation Of Revenue [Line Items]
Concentration Risk, Percentage	14.70%	19.90%	15.10%	16.70%
Engineering and Other Scientific [Member] | Time And Materials Contracts [Member]
Disaggregation Of Revenue [Line Items]
Revenues	$ 68,859	$ 59,582	$ 205,999	$ 186,486
Engineering and Other Scientific [Member] | Fixed Price Contracts [Member]
Disaggregation Of Revenue [Line Items]
Revenues	14,201	18,315	44,135	45,494
Environmental and Health [Member] | Time And Materials Contracts [Member]
Disaggregation Of Revenue [Line Items]
Revenues	17,803	16,755	54,612	52,987
Environmental and Health [Member] | Fixed Price Contracts [Member]
Disaggregation Of Revenue [Line Items]
Revenues	685	650	2,339	2,413
Sales Revenue, Net [Member] | Time And Materials Contracts [Member]
Disaggregation Of Revenue [Line Items]
Revenues	86,662	76,337	260,611	239,473
Sales Revenue, Net [Member] | Fixed Price Contracts [Member]
Disaggregation Of Revenue [Line Items]
Revenues	$ 14,886	$ 18,965	$ 46,474	$ 47,907