Segment Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 28, 2018	Sep. 29, 2017	Sep. 28, 2018	Sep. 29, 2017
Segment Reporting Information [Line Items]
Revenues	$ 95,302	$ 87,555	$ 287,380	$ 259,517
Operating Income	20,594	19,305	64,670	54,256
Capital Expenditures	4,716	828	13,665	3,354
Depreciation and Amortization	1,561	1,567	4,710	4,762
Operating segments
Segment Reporting Information [Line Items]
Operating Income	31,185	29,095	95,680	87,032
Capital Expenditures	1,921	726	4,123	2,634
Depreciation and Amortization	1,146	1,150	3,442	3,513
Engineering and Other Scientific
Segment Reporting Information [Line Items]
Revenues	77,853	70,670	231,980	207,148
Operating Income	25,784	23,645	77,499	70,279
Capital Expenditures	1,878	662	3,982	2,464
Depreciation and Amortization	1,102	1,103	3,318	3,381
Environmental and Health
Segment Reporting Information [Line Items]
Revenues	17,449	16,885	55,400	52,369
Operating Income	5,401	5,450	18,181	16,753
Capital Expenditures	43	64	141	170
Depreciation and Amortization	44	47	124	132
Corporate
Segment Reporting Information [Line Items]
Operating Income	(10,591)	(9,790)	(31,010)	(32,776)
Capital Expenditures	2,795	102	9,542	720
Depreciation and Amortization	$ 415	$ 417	$ 1,268	$ 1,249