Segment Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Oct. 01, 2021	Oct. 02, 2020	Oct. 01, 2021	Oct. 02, 2020
Segment Reporting Information [Line Items]
Revenues	$ 116,405	$ 98,663	$ 352,763	$ 296,661
Operating Income	32,119	20,407	85,549	65,515
Capital Expenditures	1,398	1,621	5,203	4,304
Depreciation and Amortization	1,645	1,689	4,943	5,176
Operating segments
Segment Reporting Information [Line Items]
Operating Income	42,527	32,401	129,911	94,151
Capital Expenditures	620	616	2,117	2,164
Depreciation and Amortization	1,138	1,091	3,215	3,389
Corporate
Segment Reporting Information [Line Items]
Operating Income	(10,408)	(11,994)	(44,362)	(28,636)
Capital Expenditures	778	1,005	3,086	2,140
Depreciation and Amortization	507	598	1,728	1,787
Engineering and Other Scientific | Operating segments
Segment Reporting Information [Line Items]
Revenues	97,100	79,230	288,848	236,239
Operating Income	36,676	25,952	108,646	73,400
Capital Expenditures	563	594	1,981	2,061
Depreciation and Amortization	1,089	1,043	3,074	3,248
Environmental and Health | Operating segments
Segment Reporting Information [Line Items]
Revenues	19,305	19,433	63,915	60,422
Operating Income	5,851	6,449	21,265	20,751
Capital Expenditures	57	22	136	103
Depreciation and Amortization	$ 49	$ 48	$ 141	$ 141