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Document and Entity Information - USD ($)
 12 Months Ended
Jan. 01, 2021
Feb. 19, 2021
Jul. 02, 2020
Cover [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Jan. 01, 2021    
Document Fiscal Year Focus 2020    
Document Fiscal Period Focus FY    
Entity Registrant Name EXPONENT, INC.    
Entity Central Index Key 0000851520    
Current Fiscal Year End Date --01-01    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Large Accelerated Filer    
Entity Public Float     $ 3,091,690,681
Trading Symbol EXPO    
Entity Common Stock, Shares Outstanding   51,803,884  
Entity Shell Company false    
Entity Emerging Growth Company false    
Entity Small Business false    
Entity File Number 0-18655    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 77-0218904    
Entity Address, Address Line One 149 Commonwealth Drive    
Entity Address, City or Town Menlo Park    
Entity Address, State or Province CA    
Entity Address, Postal Zip Code 94025    
City Area Code 650    
Local Phone Number 326-9400    
Document Annual Report true    
Document Transition Report false    
ICFR Auditor Attestation Flag true    
Entity Interactive Data Current Yes    
Title of each class Common Stock, par value $0.001 per share    
Name of each exchange on which registered NASDAQ    
Documents Incorporated by Reference

Portions of the registrant’s Definitive Proxy Statement for the registrant’s 2021 Annual Meeting of Stockholders to be held on June 3, 2021 are incorporated by reference into Part III of this Annual Report on Form 10-K.