Segment Information (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Jan. 01, 2021	Oct. 02, 2020	Jul. 03, 2020	Apr. 03, 2020	Jan. 03, 2020	Sep. 27, 2019	Jun. 28, 2019	Mar. 29, 2019	Jan. 01, 2021	Jan. 03, 2020	Dec. 28, 2018
Segment Reporting Information [Line Items]
Revenues									$ 399,900,000	$ 417,199,000	$ 379,523,000
Operating Income	$ 17,734,000	$ 20,407,000	$ 9,120,000	$ 35,988,000	$ 21,350,000	$ 23,184,000	$ 24,823,000	$ 15,754,000	83,249,000	85,111,000	91,456,000
Capital Expenditures									5,107,000	22,290,000	17,381,000
Depreciation and Amortization									6,871,000	6,806,000	6,292,000
Operating segments
Segment Reporting Information [Line Items]
Operating Income									127,344,000	137,411,000	124,131,000
Capital Expenditures									2,339,000	4,779,000	4,727,000
Depreciation and Amortization									4,435,000	5,033,000	4,606,000
Corporate
Segment Reporting Information [Line Items]
Operating Income									(44,095,000)	(52,300,000)	(32,675,000)
Capital Expenditures									2,768,000	17,511,000	12,654,000
Depreciation and Amortization									2,436,000	1,773,000	1,686,000
Engineering and Other Scientific | Operating segments
Segment Reporting Information [Line Items]
Revenues									319,346,000	339,796,000	306,265,000
Operating Income									100,616,000	110,822,000	100,307,000
Capital Expenditures									2,237,000	4,675,000	4,528,000
Depreciation and Amortization									4,239,000	4,827,000	4,435,000
Environmental and Health | Operating segments
Segment Reporting Information [Line Items]
Revenues									80,554,000	77,403,000	73,258,000
Operating Income									26,728,000	26,589,000	23,824,000
Capital Expenditures									102,000	104,000	199,000
Depreciation and Amortization									$ 196,000	$ 206,000	$ 171,000