Segment Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 27, 2019	Sep. 28, 2018	Sep. 27, 2019	Sep. 28, 2018
Segment Reporting Information [Line Items]
Revenues	$ 101,548	$ 95,302	$ 307,085	$ 287,380
Operating Income	23,184	20,594	63,761	64,670
Capital Expenditures	4,878	4,716	18,119	13,665
Depreciation and Amortization	1,672	1,561	4,904	4,710
Operating segments
Segment Reporting Information [Line Items]
Operating Income	34,100	31,185	102,561	95,680
Capital Expenditures	681	1,921	3,444	4,123
Depreciation and Amortization	1,234	1,146	3,618	3,442
Corporate
Segment Reporting Information [Line Items]
Operating Income	(10,916)	(10,591)	(38,800)	(31,010)
Capital Expenditures	4,197	2,795	14,675	9,542
Depreciation and Amortization	438	415	1,286	1,268
Engineering and Other Scientific | Operating segments
Segment Reporting Information [Line Items]
Revenues	83,060	77,853	250,134	231,980
Operating Income	27,925	25,784	83,151	77,499
Capital Expenditures	664	1,878	3,371	3,982
Depreciation and Amortization	1,179	1,102	3,470	3,318
Environmental and Health | Operating segments
Segment Reporting Information [Line Items]
Revenues	18,488	17,449	56,951	55,400
Operating Income	6,175	5,401	19,410	18,181
Capital Expenditures	17	43	73	141
Depreciation and Amortization	$ 55	$ 44	$ 148	$ 124