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, 2018	$ 1,279	$ 54		$ 1,417	$ (317)	$ 1,154	$ 125
Beginning 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
Net income (loss)	286			250		250	36
Total other comprehensive income (loss)	69				62	62	7
Cash dividends declared to noncontrolling interests	(23)						(23)
Cash dividends paid to common stockholders, per share	(80)			(80)		(80)
Issuance of stock under equity compensation plans	6		6			6
Issuance of stock under equity compensation plans, Shares		317
Amortization of share-based compensation	21		21			21
Purchase and retirement of common stock	(3)		(3)			(3)
Purchase and retirement of common stock, Shares		(57)
Ending Balance at Sep. 30, 2021	$ 1,090	$ 53	$ 24	$ 1,159	$ (289)	$ 947	$ 143
Ending Balance, Shares at Sep. 30, 2021		56,727