Segment Information (Detail) - USD ($)	12 Months Ended
	Dec. 29, 2023	Dec. 30, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Revenues	$ 536,766,000	$ 513,293,000	$ 466,269,000
Operating Income	111,322,000	140,842,000	108,927,000
Capital Expenditures	18,695,000	12,647,000	6,637,000
Depreciation and Amortization	8,916,000	7,079,000	6,487,000
Operating segments
Segment Reporting Information [Line Items]
Operating Income	182,350,000	180,019,000	168,352,000
Capital Expenditures	4,065,000	4,835,000	2,952,000
Depreciation and Amortization	6,300,000	4,660,000	4,224,000
Corporate
Segment Reporting Information [Line Items]
Operating Income	(71,028,000)	(39,177,000)	(59,425,000)
Capital Expenditures	14,630,000	7,812,000	3,685,000
Depreciation and Amortization	2,616,000	2,419,000	2,263,000
Engineering and Other Scientific | Operating segments
Segment Reporting Information [Line Items]
Revenues	446,888,000	427,796,000	380,909,000
Operating Income	153,918,000	152,679,000	140,400,000
Capital Expenditures	3,895,000	4,661,000	2,792,000
Depreciation and Amortization	6,087,000	4,489,000	4,031,000
Environmental and Health | Operating segments
Segment Reporting Information [Line Items]
Revenues	89,878,000	85,497,000	85,360,000
Operating Income	28,432,000	27,340,000	27,952,000
Capital Expenditures	170,000	174,000	160,000
Depreciation and Amortization	$ 213,000	$ 171,000	$ 193,000