Segment Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2017	Sep. 30, 2016	Sep. 29, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Revenues	$ 87,555	$ 77,612	$ 259,517	$ 238,063
Operating Income	19,305	15,595	54,256	46,962
Capital Expenditures	828	9,681	3,354	13,063
Depreciation and Amortization	1,567	1,582	4,762	4,509
Operating segments
Segment Reporting Information [Line Items]
Operating Income	29,095	24,691	87,032	74,613
Capital Expenditures	726	935	2,634	3,459
Depreciation and Amortization	1,150	1,181	3,513	3,406
Engineering and Other Scientific
Segment Reporting Information [Line Items]
Revenues	70,670	61,237	207,148	187,026
Operating Income	23,645	19,933	70,279	60,632
Capital Expenditures	662	902	2,464	3,365
Depreciation and Amortization	1,103	1,136	3,381	3,273
Environmental and Health
Segment Reporting Information [Line Items]
Revenues	16,885	16,375	52,369	51,037
Operating Income	5,450	4,758	16,753	13,981
Capital Expenditures	64	33	170	94
Depreciation and Amortization	47	45	132	133
Corporate
Segment Reporting Information [Line Items]
Operating Income	(9,790)	(9,096)	(32,776)	(27,651)
Capital Expenditures	102	8,746	720	9,604
Depreciation and Amortization	$ 417	$ 401	$ 1,249	$ 1,103