Segment Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 29, 2019	Mar. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 99,031	$ 96,457
Operating Income	15,754	21,598
Capital Expenditures	6,341	6,810
Depreciation and Amortization	1,590	1,555
Operating segments
Segment Reporting Information [Line Items]
Operating Income	32,170	32,537
Capital Expenditures	1,821	864
Depreciation and Amortization	1,166	1,134
Engineering and Other Scientific
Segment Reporting Information [Line Items]
Revenues	80,254	77,047
Operating Income	25,974	25,913
Capital Expenditures	1,778	813
Depreciation and Amortization	1,121	1,095
Environmental and Health
Segment Reporting Information [Line Items]
Revenues	18,777	19,410
Operating Income	6,196	6,624
Capital Expenditures	43	51
Depreciation and Amortization	45	39
Corporate
Segment Reporting Information [Line Items]
Operating Income	(16,416)	(10,939)
Capital Expenditures	4,520	5,946
Depreciation and Amortization	$ 424	$ 421