Segment Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Oct. 01, 2021	Sep. 30, 2022	Oct. 01, 2021
Segment Reporting Information [Line Items]
Revenues	$ 127,179	$ 116,405	$ 385,938	$ 352,763
Operating Income	36,813	32,119	118,807	85,549
Capital Expenditures	2,090	1,398	9,541	5,203
Depreciation and Amortization	1,723	1,645	5,224	4,943
Operating segments
Segment Reporting Information [Line Items]
Operating Income	45,763	42,527	138,966	129,911
Capital Expenditures	918	620	3,154	2,117
Depreciation and Amortization	1,149	1,138	3,425	3,215
Corporate
Segment Reporting Information [Line Items]
Operating Income	(8,950)	(10,408)	(20,159)	(44,362)
Capital Expenditures	1,172	778	6,387	3,086
Depreciation and Amortization	574	507	1,799	1,728
Engineering and Other Scientific | Operating segments
Segment Reporting Information [Line Items]
Revenues	107,403	97,100	321,168	288,848
Operating Income	39,385	36,676	117,907	108,646
Capital Expenditures	870	563	3,050	1,981
Depreciation and Amortization	1,108	1,089	3,301	3,074
Environmental and Health | Operating segments
Segment Reporting Information [Line Items]
Revenues	19,776	19,305	64,770	63,915
Operating Income	6,378	5,851	21,059	21,265
Capital Expenditures	48	57	104	136
Depreciation and Amortization	$ 41	$ 49	$ 124	$ 141