Document and Entity Information - shares	6 Months Ended
	Mar. 31, 2017	May 04, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q2
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		62,475,249