Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - 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, 2022	$ 1,032	$ 52	$ 1	$ 1,284	$ (439)	$ 898	$ 134
Beginning Balance, Shares at Sep. 30, 2022		56,249
Net income (loss)	66			54		54	12
Total other comprehensive income (loss)	87				81	81	6
Cash dividends paid, Common stock, per share	(21)			(21)		(21)
Cash dividends declared to noncontrolling interests	(2)						(2)
Issuance of stock under equity compensation plans	3	$ 1	2			3
Issuance of stock under equity compensation plans, Shares		309
Share-based compensation	11		11			11
Purchase and retirement of common stock	(17)		(14)	(3)		(17)
Purchase and retirement of common stock, Shares		(242)
Ending Balance at Dec. 31, 2022	1,159	$ 53		1,314	(358)	1,009	150
Ending Balance, Shares at Dec. 31, 2022		56,316
Beginning Balance at Sep. 30, 2023	1,407	$ 52		1,574	(362)	1,264	143
Beginning Balance, Shares at Sep. 30, 2023		55,244
Net income (loss)	61			50		50	11
Total other comprehensive income (loss)	61				57	57	4
Cash dividends paid, Common stock, per share	(22)			(22)		(22)
Cash dividends declared to noncontrolling interests	(12)						(12)
Issuance of stock under equity compensation plans	7		7			7
Issuance of stock under equity compensation plans, Shares		441
Share-based compensation	6		6			6
Purchase and retirement of common stock	(33)		$ (13)	(20)		(33)
Purchase and retirement of common stock, Shares		(260)
Ending Balance at Dec. 31, 2023	$ 1,475	$ 52		$ 1,582	$ (305)	$ 1,329	$ 146
Ending Balance, Shares at Dec. 31, 2023		55,425