Segment Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Oct. 02, 2015	Sep. 30, 2016	Oct. 02, 2015
Segment Reporting Information [Line Items]
Revenues	$ 77,612	$ 78,994	$ 238,063	$ 239,151
Operating Income	15,595	20,921	46,962	54,654
Capital Expenditures	9,681	2,641	13,063	4,355
Depreciation and Amortization	1,582	1,329	4,509	4,034
Operating segments
Segment Reporting Information [Line Items]
Operating Income	24,691	25,291	74,613	76,739
Capital Expenditures	935	1,109	3,459	2,391
Depreciation and Amortization	1,181	997	3,406	3,010
Engineering and Other Scientific
Segment Reporting Information [Line Items]
Revenues	61,237	61,052	187,026	180,212
Operating Income	19,933	20,252	60,632	58,605
Capital Expenditures	902	1,057	3,365	2,268
Depreciation and Amortization	1,136	951	3,273	2,877
Environmental and Health
Segment Reporting Information [Line Items]
Revenues	16,375	17,942	51,037	58,939
Operating Income	4,758	5,039	13,981	18,134
Capital Expenditures	33	52	94	123
Depreciation and Amortization	45	46	133	133
Corporate
Segment Reporting Information [Line Items]
Operating Income	(9,096)	(4,370)	(27,651)	(22,085)
Capital Expenditures	8,746	1,532	9,604	1,964
Depreciation and Amortization	$ 401	$ 332	$ 1,103	$ 1,024