Segment Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Oct. 02, 2020	Sep. 27, 2019	Oct. 02, 2020	Sep. 27, 2019
Segment Reporting Information [Line Items]
Revenues	$ 98,663	$ 101,548	$ 296,661	$ 307,085
Operating Income	20,407	23,184	65,515	63,761
Capital Expenditures	1,621	4,878	4,304	18,119
Depreciation and Amortization	1,689	1,672	5,176	4,904
Operating segments
Segment Reporting Information [Line Items]
Operating Income	32,401	34,100	94,151	102,561
Capital Expenditures	616	681	2,164	3,444
Depreciation and Amortization	1,091	1,234	3,389	3,618
Corporate
Segment Reporting Information [Line Items]
Operating Income	(11,994)	(10,916)	(28,636)	(38,800)
Capital Expenditures	1,005	4,197	2,140	14,675
Depreciation and Amortization	598	438	1,787	1,286
Engineering and Other Scientific | Operating segments
Segment Reporting Information [Line Items]
Revenues	79,230	83,060	236,239	250,134
Operating Income	25,952	27,925	73,400	83,151
Capital Expenditures	594	664	2,061	3,371
Depreciation and Amortization	1,043	1,179	3,248	3,470
Environmental and Health | Operating segments
Segment Reporting Information [Line Items]
Revenues	19,433	18,488	60,422	56,951
Operating Income	6,449	6,175	20,751	19,410
Capital Expenditures	22	17	103	73
Depreciation and Amortization	$ 48	$ 55	$ 141	$ 148