Segment Information (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 30, 2016	Sep. 30, 2016	Jul. 01, 2016	Apr. 01, 2016	Jan. 01, 2016	Oct. 02, 2015	Jul. 03, 2015	Apr. 03, 2015	Dec. 30, 2016		Jan. 01, 2016		Jan. 02, 2015
Segment Reporting Information [Line Items]
Revenues	$ 77,013	$ 77,612	$ 77,295	$ 83,156	$ 73,681	$ 78,994	$ 79,864	$ 80,293	$ 315,076	[1]	$ 312,832	[1]	$ 304,704	[1]
Operating Income	$ 14,949	$ 15,595	$ 14,931	$ 16,436	$ 14,279	$ 20,921	$ 18,705	$ 15,028	61,911		68,933		63,549
Capital Expenditures									14,393		5,379		4,947
Depreciation and Amortization									6,131		5,479		5,404
Operating segments
Segment Reporting Information [Line Items]
Operating Income									99,144		98,627		97,352
Capital Expenditures									4,433		3,361		3,930
Depreciation and Amortization									4,610		4,101		3,834
Engineering and Other Scientific
Segment Reporting Information [Line Items]
Revenues									248,297		237,959		223,384
Operating Income									80,494		76,817		72,207
Capital Expenditures									4,309		3,197		3,719
Depreciation and Amortization									4,429		3,919		3,637
Environmental and Health
Segment Reporting Information [Line Items]
Revenues									66,779		74,873		81,320
Operating Income									18,650		21,810		25,145
Capital Expenditures									124		164		211
Depreciation and Amortization									181		182		197
Corporate
Segment Reporting Information [Line Items]
Operating Income									(37,233)		(29,694)		(33,803)
Capital Expenditures									9,960		2,018		1,017
Depreciation and Amortization									$ 1,521		$ 1,378		$ 1,570

[1] Geographic revenues are allocated based on the location of the client.