Segment Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Oct. 02, 2015	Oct. 03, 2014	Oct. 02, 2015	Oct. 03, 2014
Segment Reporting Information [Line Items]
Revenues	$ 78,994	$ 78,557	$ 239,151	$ 231,093
Operating Income	20,921	19,238	54,654	49,416
Capital Expenditures	2,641	1,930	4,355	3,906
Depreciation and Amortization	1,329	1,346	4,034	3,979
Engineering and Other Scientific
Segment Reporting Information [Line Items]
Revenues	61,052	57,491	180,212	168,611
Operating Income	20,252	18,939	58,605	55,457
Capital Expenditures	1,057	1,509	2,268	2,833
Depreciation and Amortization	951	923	2,877	2,664
Environmental and Health
Segment Reporting Information [Line Items]
Revenues	17,942	21,066	58,939	62,482
Operating Income	5,039	7,007	18,134	20,171
Capital Expenditures	52	36	123	151
Depreciation and Amortization	46	49	133	145
Operating segments
Segment Reporting Information [Line Items]
Operating Income	25,291	25,946	76,739	75,628
Capital Expenditures	1,109	1,545	2,391	2,984
Depreciation and Amortization	997	972	3,010	2,809
Corporate
Segment Reporting Information [Line Items]
Operating Income	(4,370)	(6,708)	(22,085)	(26,212)
Capital Expenditures	1,532	385	1,964	922
Depreciation and Amortization	$ 332	$ 374	$ 1,024	$ 1,170