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, 2015	$ 1,357	$ 55		$ 1,497	$ (299)	$ 1,253	$ 104
Beginning Balance, Shares at Sep. 30, 2015		62,458
Net income (loss) attributable to Cabot Corporation	147			147		147
Net income (loss) attributable to noncontrolling interests, net of tax	15						15
Total other comprehensive income (loss)	(31)				(26)	(26)	(5)
Cash dividends paid/declared 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	1,389	$ 55		1,561	(325)	1,291	98
Ending Balance, Shares at Sep. 30, 2016		62,211
Net income (loss) attributable to Cabot Corporation	248			248		248
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/declared 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	1,625	$ 56		1,707	(259)	1,504	121
Ending Balance, Shares at Sep. 30, 2017		61,884
Net income (loss) attributable to Cabot Corporation	(113)			(113)		(113)
Net income (loss) attributable to noncontrolling interests, net of tax	39						39
Total other comprehensive income (loss)	(62)				(58)	(58)	(4)
Acquisition of noncontrolling interest			(1)			(1)	1
Cash dividends paid/declared to noncontrolling interests	(32)						(32)
Cash dividends paid to common stockholders	(80)			(80)		(80)
Issuance of stock under equity compensation plans	22		22			22
Issuance of stock under equity compensation plans, Shares		733
Amortization of share-based compensation	22		22			22
Purchase and retirement of common stock	(142)	$ (2)	$ (43)	(97)		(142)
Purchase and retirement of common stock, Shares		(2,250)
Ending Balance at Sep. 30, 2018	$ 1,279	$ 54		$ 1,417	$ (317)	$ 1,154	$ 125
Ending Balance, Shares at Sep. 30, 2018		60,367