Segment Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Oct. 03, 2014	Sep. 27, 2013	Oct. 03, 2014	Sep. 27, 2013
Segment Reporting Information [Line Items]
Revenues	$ 78,557	$ 75,231	$ 231,093	$ 223,396
Operating income	19,238	15,160	49,416	43,604
Capital expenditures	1,930	892	3,906	4,593
Depreciation and amortization	1,346	1,283	3,979	3,667
Engineering and Other Scientific
Segment Reporting Information [Line Items]
Revenues	57,491	56,395	168,611	163,353
Operating income	18,939	18,115	55,457	51,747
Capital expenditures	1,509	664	2,833	3,924
Depreciation and amortization	923	813	2,664	2,269
Environmental and Health
Segment Reporting Information [Line Items]
Revenues	21,066	18,836	62,482	60,043
Operating income	7,007	5,624	20,171	19,104
Capital expenditures	36	41	151	110
Depreciation and amortization	49	76	145	221
Operating segments
Segment Reporting Information [Line Items]
Operating income	25,946	23,739	75,628	70,851
Capital expenditures	1,545	705	2,984	4,034
Depreciation and amortization	972	889	2,809	2,490
Corporate
Segment Reporting Information [Line Items]
Operating income	(6,708)	(8,579)	(26,212)	(27,247)
Capital expenditures	385	187	922	559
Depreciation and amortization	$ 374	$ 394	$ 1,170	$ 1,177