Stockholders' Equity - Additional Information (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended				12 Months Ended			0 Months Ended	1 Months Ended
	Dec. 28, 2012	Dec. 30, 2011	Dec. 31, 2010	May 26, 2006	Dec. 28, 2012 Treasury Stock	Dec. 30, 2011 Treasury Stock	Dec. 31, 2010 Treasury Stock	Feb. 09, 2012 Exponent Inc	May 25, 2011 Exponent Inc
Class of Stock [Line Items]
Preferred stock, shares authorized	5,000	5,000
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock shares authorized limitation on use				2,000
Net losses related to the re-issuance of treasury stock	$ 10,600	$ 9,349	$ 11,041
Purchase of treasury shares (in shares)					480	1,002	244
Purchase of treasury shares	(23,395)	(40,573)	(6,899)		(23,395)	(40,573)	(6,899)
Stock repurchase program, authorized amount								35,000	35,000
Stock repurchase program, remaining authorized repurchase amount	$ 21,011