Document and Entity Information	3 Months Ended
	Dec. 31, 2014	Feb. 03, 2015
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Dec. 31, 2014
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Trading Symbol	CBT
Entity Registrant Name	CABOT CORP
Entity Central Index Key	0000016040
Current Fiscal Year End Date	--09-30
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		63,717,924