Document And Entity Information	9 Months Ended
	Oct. 03, 2014	Oct. 31, 2014
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Oct. 03, 2014
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	Q3
Entity Registrant Name	EXPONENT INC
Entity Central Index Key	0000851520
Current Fiscal Year End Date	--01-02
Entity Filer Category	Accelerated Filer
Trading Symbol	EXPO
Entity Common Stock, Shares Outstanding		12,866,600