Consolidated Statement of Changes in Stockholders' Equity - USD ($) shares in Thousands, $ in Millions	Total	Common Stock, Net of Treasury Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Total Cabot Corporation Stockholders' Equity [Member]	Noncontrolling Interests [Member]
Beginning Balance at Sep. 30, 2014	$ 2,064	$ 57	$ 49	$ 1,900	$ (64)	$ 1,942	$ 122
Beginning Balance, Shares at Sep. 30, 2014		64,383
Net income (loss) attributable to Cabot Corporation	(334)			(334)		(334)
Net income (loss) attributable to noncontrolling interests, net of tax	8						8
Total other comprehensive income (loss)	(239)				(235)	(235)	(4)
Cash dividends paid to noncontrolling interests	(22)						(22)
Cash dividends paid to common stockholders	(56)			(56)		(56)
Issuance of stock under equity compensation plans	6		6			6
Issuance of stock under equity compensation plans, Shares		450
Amortization of share-based compensation	12		12			12
Purchase and retirement of common stock	(101)	$ (2)	(67)	(32)		(101)
Purchase and retirement of common stock, Shares		(2,375)
Ending Balance at Sep. 30, 2015	1,338	$ 55		1,478	(299)	1,234	104
Ending Balance, Shares at Sep. 30, 2015		62,458
Net income (loss) attributable to Cabot Corporation	149			149		149
Net income (loss) attributable to noncontrolling interests, net of tax	15						15
Total other comprehensive income (loss)	(31)				(26)	(26)	(5)
Cash dividends paid to noncontrolling interests	(16)						(16)
Cash dividends paid to common stockholders	(65)			(65)		(65)
Issuance of stock under equity compensation plans	10	$ 1	9			10
Issuance of stock under equity compensation plans, Shares		737
Amortization of share-based compensation	17		17			17
Purchase and retirement of common stock	(45)	$ (1)	(26)	(18)		(45)
Purchase and retirement of common stock, Shares		(984)
Ending Balance at Sep. 30, 2016	1,372	$ 55		1,544	(325)	1,274	98
Ending Balance, Shares at Sep. 30, 2016		62,211
Net income (loss) attributable to Cabot Corporation	241			241		241
Net income (loss) attributable to noncontrolling interests, net of tax	25						25
Total other comprehensive income (loss)	68				66	66	2
Contributions from noncontrolling interest	4						4
Acquisition of noncontrolling interest			(6)			(6)	6
Cash dividends paid to noncontrolling interests	(14)						(14)
Cash dividends paid to common stockholders	(77)			(77)		(77)
Issuance of stock under equity compensation plans	27	$ 2	25			27
Issuance of stock under equity compensation plans, Shares		833
Amortization of share-based compensation	16		16			16
Purchase and retirement of common stock	(61)	$ (1)	$ (35)	(25)		(61)
Purchase and retirement of common stock, Shares		(1,160)
Ending Balance at Sep. 30, 2017	$ 1,601	$ 56		$ 1,683	$ (259)	$ 1,480	$ 121
Ending Balance, Shares at Sep. 30, 2017		61,884