Document And Entity Information - USD ($)	12 Months Ended
	Dec. 28, 2018	Feb. 15, 2019	Jun. 29, 2018
Document Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 28, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Entity Registrant Name	EXPONENT INC
Entity Central Index Key	0000851520
Current Fiscal Year End Date	--12-28
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 2,136,924,922
Trading Symbol	EXPO
Entity Common Stock, Shares Outstanding		51,498,369
Entity Shell Company	false
Entity Emerging Growth Company	false
Entity Small Business	false