Segment Reporting (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 29, 2018	Jun. 30, 2017	Jun. 29, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 95,621	$ 87,840	$ 192,078	$ 171,962
Operating Income	22,478	20,317	44,076	34,951
Capital Expenditures	2,139	1,219	8,949	2,526
Depreciation and Amortization	1,594	1,629	3,149	3,195
Operating segments
Segment Reporting Information [Line Items]
Operating Income	31,958	30,137	64,495	57,937
Capital Expenditures	1,338	858	2,202	1,908
Depreciation and Amortization	1,162	1,207	2,296	2,363
Engineering and Other Scientific
Segment Reporting Information [Line Items]
Revenues	77,080	69,795	154,127	136,478
Operating Income	25,802	24,313	51,715	46,634
Capital Expenditures	1,291	794	2,104	1,802
Depreciation and Amortization	1,121	1,163	2,216	2,278
Environmental and Health
Segment Reporting Information [Line Items]
Revenues	18,541	18,045	37,951	35,484
Operating Income	6,156	5,824	12,780	11,303
Capital Expenditures	47	64	98	106
Depreciation and Amortization	41	44	80	85
Corporate
Segment Reporting Information [Line Items]
Operating Income	(9,480)	(9,820)	(20,419)	(22,986)
Capital Expenditures	801	361	6,747	618
Depreciation and Amortization	$ 432	$ 422	$ 853	$ 832