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Stockholders' Equity - Additional Information (Detail) (USD $)
 1 Months Ended 12 Months Ended 0 Months Ended
Feb. 28, 2014
Jan. 03, 2014
Dec. 28, 2012
Dec. 30, 2011
May 23, 2006
Feb. 15, 2013
Exponent Inc [Member]
Feb. 09, 2012
Exponent Inc [Member]
Class of Stock [Line Items]              
Preferred Stock, Shares Authorized   5,000,000 5,000,000        
Preferred stock, par value   $ 0.001 $ 0.001        
Preferred Stock Shares Authorized Limitation On Use         2,000,000    
Net losses related to the re-issuance of treasury stock   $ 10,383,000 $ 10,600,000 $ 9,349,000      
Purchase of treasury shares (in shares)   438,000 480,000 1,002,000      
Purchase of treasury shares   25,011,000 23,395,000 40,573,000      
Stock repurchase program, authorized amount           35,000,000 35,000,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount   $ 31,000,000          
Dividends Payable, Date Declared Feb. 05, 2014            
Common Stock, Dividends, Per Share, Declared $ 0.25            
Dividends Payable, Date to be Paid Mar. 28, 2014            
Dividends Payable, Date of Record Mar. 07, 2014            
Preferred stock, shares issued   0 0        
Preferred stock, shares outstanding   0 0