Consolidated Statements 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, 2017	$ 1,625	$ 56		$ 1,707	$ (259)	$ 1,504	$ 121
Beginning Balance, Shares at Sep. 30, 2017		61,884
Net income (loss)	(74)			(113)		(113)	39
Total other comprehensive income (loss)	(62)				(58)	(58)	(4)
Acquisition of noncontrolling interest			$ (1)			(1)	1
Cash dividends declared to noncontrolling interests	(32)						(32)
Cash dividends paid to common stockholders, per share	(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
Net income (loss)	186			157		157	29
Total other comprehensive income (loss)	(80)				(74)	(74)	(6)
Acquisition of noncontrolling interest			(1)			(1)	1
Cash dividends declared to noncontrolling interests	(13)						(13)
Cash dividends paid to common stockholders, per share	(80)			(80)		(80)
Issuance of stock under equity compensation plans	4	$ 2	2			4
Issuance of stock under equity compensation plans, Shares		483
Amortization of share-based compensation	11		11			11
Purchase and retirement of common stock	(173)	$ (4)	(12)	(157)		(173)
Purchase and retirement of common stock, Shares		(3,769)
Ending Balance at Sep. 30, 2019	1,134	$ 52		1,337	(391)	998	136
Ending Balance, Shares at Sep. 30, 2019		57,081
Net income (loss)	(221)			(238)		(238)	17
Adoption of accounting standards				3	(3)
Total other comprehensive income (loss)	48				43	43	5
Cash dividends declared to noncontrolling interests	(35)						(35)
Cash dividends paid to common stockholders, per share	(80)			(80)		(80)
Issuance of stock under equity compensation plans	3	$ 2	1			3
Issuance of stock under equity compensation plans, Shares		330
Amortization of share-based compensation	9		9			9
Purchase and retirement of common stock	(44)	$ (1)	$ (10)	(33)		(44)
Purchase and retirement of common stock, Shares		(944)
Ending Balance at Sep. 30, 2020	$ 814	$ 53		$ 989	$ (351)	$ 691	$ 123
Ending Balance, Shares at Sep. 30, 2020		56,467