Segment Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Apr. 01, 2016
Segment Reporting Information [Line Items]
Revenues	$ 84,122	$ 83,156
Operating Income	14,634	16,436
Capital Expenditures	1,307	1,481
Depreciation and Amortization	1,566	1,403
Operating segments
Segment Reporting Information [Line Items]
Operating Income	27,800	26,333
Capital Expenditures	1,050	1,213
Depreciation and Amortization	1,156	1,064
Engineering and Other Scientific
Segment Reporting Information [Line Items]
Revenues	66,683	64,810
Operating Income	22,321	21,138
Capital Expenditures	1,008	1,190
Depreciation and Amortization	1,115	1,021
Environmental and Health
Segment Reporting Information [Line Items]
Revenues	17,439	18,346
Operating Income	5,479	5,195
Capital Expenditures	42	23
Depreciation and Amortization	41	43
Corporate
Segment Reporting Information [Line Items]
Operating Income	(13,166)	(9,897)
Capital Expenditures	257	268
Depreciation and Amortization	$ 410	$ 339