Revenue Recognition- Summary of Revenues (Detail)	3 Months Ended		6 Months Ended
	Jul. 02, 2021	Jul. 03, 2020	Jul. 02, 2021	Jul. 03, 2020
Disaggregation Of Revenue [Line Items]
Concentration Risk, Percentage	14.00%	11.00%	12.00%
Time And Materials Contracts [Member] | Sales Revenue, Net [Member] | Revenue from Rights Concentration Risk [Member]
Disaggregation Of Revenue [Line Items]
Concentration Risk, Percentage	79.00%	81.00%	79.00%	83.00%
Time And Materials Contracts [Member] | Engineering and Other Scientific [Member] | Sales Revenue, Net [Member] | Revenue from Rights Concentration Risk [Member]
Disaggregation Of Revenue [Line Items]
Concentration Risk, Percentage	62.00%	60.00%	61.00%	63.00%
Time And Materials Contracts [Member] | Environmental and Health [Member] | Sales Revenue, Net [Member] | Revenue from Rights Concentration Risk [Member]
Disaggregation Of Revenue [Line Items]
Concentration Risk, Percentage	17.00%	21.00%	18.00%	20.00%
Fixed Price Contracts [Member] | Sales Revenue, Net [Member] | Revenue from Rights Concentration Risk [Member]
Disaggregation Of Revenue [Line Items]
Concentration Risk, Percentage	21.00%	19.00%	21.00%	17.00%
Fixed Price Contracts [Member] | Engineering and Other Scientific [Member] | Sales Revenue, Net [Member] | Revenue from Rights Concentration Risk [Member]
Disaggregation Of Revenue [Line Items]
Concentration Risk, Percentage	20.00%	18.00%	20.00%	16.00%
Fixed Price Contracts [Member] | Environmental and Health [Member] | Sales Revenue, Net [Member] | Revenue from Rights Concentration Risk [Member]
Disaggregation Of Revenue [Line Items]
Concentration Risk, Percentage	1.00%	1.00%	1.00%	1.00%