Segment Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 02, 2021	Jul. 03, 2020	Jul. 02, 2021	Jul. 03, 2020
Segment Reporting Information [Line Items]
Revenues	$ 119,877	$ 92,045	$ 236,358	$ 197,998
Operating Income	29,372	9,120	53,430	45,108
Capital Expenditures	1,499	1,644	3,805	2,683
Depreciation and Amortization	1,642	1,701	3,298	3,487
Operating segments
Segment Reporting Information [Line Items]
Operating Income	45,409	27,856	87,384	61,750
Capital Expenditures	844	920	1,497	1,548
Depreciation and Amortization	1,023	1,114	2,079	2,298
Corporate
Segment Reporting Information [Line Items]
Operating Income	(16,037)	(18,736)	(33,954)	(16,642)
Capital Expenditures	655	724	2,308	1,135
Depreciation and Amortization	619	587	1,219	1,189
Engineering and Other Scientific | Operating segments
Segment Reporting Information [Line Items]
Revenues	98,139	72,123	191,748	157,009
Operating Income	37,914	20,807	71,970	47,448
Capital Expenditures	814	901	1,418	1,467
Depreciation and Amortization	976	1,068	1,986	2,205
Environmental and Health | Operating segments
Segment Reporting Information [Line Items]
Revenues	21,738	19,922	44,610	40,989
Operating Income	7,495	7,049	15,414	14,302
Capital Expenditures	30	19	79	81
Depreciation and Amortization	$ 47	$ 46	$ 93	$ 93