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Document And Entity Information
 9 Months Ended
Sep. 27, 2013
Oct. 25, 2013
Document Information [Line Items]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 27, 2013  
Document Fiscal Year Focus 2013  
Document Fiscal Period Focus Q3  
Trading Symbol EXPO  
Entity Common Stock, Shares Outstanding   13,103,213
Entity Registrant Name EXPONENT INC  
Entity Central Index Key 0000851520  
Current Fiscal Year End Date --01-03  
Entity Filer Category Accelerated Filer