Segment Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 30, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 96,457	$ 84,122
Operating Income	21,598	14,634
Capital Expenditures	6,810	1,307
Depreciation and Amortization	1,555	1,566
Operating segments
Segment Reporting Information [Line Items]
Operating Income	32,537	27,800
Capital Expenditures	864	1,050
Depreciation and Amortization	1,134	1,156
Engineering and Other Scientific
Segment Reporting Information [Line Items]
Revenues	77,047	66,683
Operating Income	25,913	22,321
Capital Expenditures	813	1,008
Depreciation and Amortization	1,095	1,115
Environmental and Health
Segment Reporting Information [Line Items]
Revenues	19,410	17,439
Operating Income	6,624	5,479
Capital Expenditures	51	42
Depreciation and Amortization	39	41
Corporate
Segment Reporting Information [Line Items]
Operating Income	(10,939)	(13,166)
Capital Expenditures	5,946	257
Depreciation and Amortization	$ 421	$ 410