Document And Entity Information - shares	6 Months Ended
	Jul. 03, 2015	Jul. 31, 2015
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jul. 03, 2015
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q2
Entity Registrant Name	EXPONENT INC
Entity Central Index Key	0000851520
Current Fiscal Year End Date	--01-01
Entity Filer Category	Large Accelerated Filer
Trading Symbol	EXPO
Entity Common Stock, Shares Outstanding		25,922,828