Document And Entity Information - USD ($)	12 Months Ended
	Jan. 01, 2016	Feb. 19, 2016	Jul. 02, 2015
Document Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Jan. 01, 2016
Document Fiscal Year Focus	2015
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			$ 996,092,846
Trading Symbol	EXPO
Entity Common Stock, Shares Outstanding		25,720,598