Segment Information (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Jan. 03, 2020	Sep. 27, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Sep. 28, 2018	Jun. 29, 2018	Mar. 30, 2018	Jan. 03, 2020	Dec. 28, 2018	Dec. 29, 2017
Segment Reporting Information [Line Items]
Revenues									$ 417,199,000	$ 379,523,000	$ 347,799,000
Operating Income	$ 21,350,000	$ 23,184,000	$ 24,823,000	$ 15,754,000	$ 26,786,000	$ 20,594,000	$ 22,478,000	$ 21,598,000	85,111,000	91,456,000	72,051,000
Capital Expenditures									22,290,000	17,381,000	4,725,000
Depreciation and Amortization									6,806,000	6,292,000	6,285,000
Operating segments
Segment Reporting Information [Line Items]
Operating Income									137,411,000	124,131,000	115,791,000
Capital Expenditures									4,779,000	4,727,000	3,866,000
Depreciation and Amortization									5,033,000	4,606,000	4,628,000
Corporate
Segment Reporting Information [Line Items]
Operating Income									(52,300,000)	(32,675,000)	(43,740,000)
Capital Expenditures									17,511,000	12,654,000	859,000
Depreciation and Amortization									1,773,000	1,686,000	1,657,000
Engineering and Other Scientific | Operating segments
Segment Reporting Information [Line Items]
Revenues									339,796,000	306,265,000	277,603,000
Operating Income									110,822,000	100,307,000	93,451,000
Capital Expenditures									4,675,000	4,528,000	3,648,000
Depreciation and Amortization									4,827,000	4,435,000	4,449,000
Environmental and Health | Operating segments
Segment Reporting Information [Line Items]
Revenues									77,403,000	73,258,000	70,196,000
Operating Income									26,589,000	23,824,000	22,340,000
Capital Expenditures									104,000	199,000	218,000
Depreciation and Amortization									$ 206,000	$ 171,000	$ 179,000