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, 2023	$ 1,407	$ 52	$ 0	$ 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
Beginning Balance at Sep. 30, 2024	1,590	$ 51		1,734	(360)	1,425	165
Beginning Balance, Shares at Sep. 30, 2024		54,297
Net income (loss)	104			93		93	11
Total other comprehensive income (loss)	(104)				(97)	(97)	(7)
Cash dividends paid, Common stock, per share	(24)			(24)		(24)
Cash dividends declared to noncontrolling interests	(20)						(20)
Issuance of stock under equity compensation plans	2		2			2
Issuance of stock under equity compensation plans, Shares		308
Share-based compensation	8		8			8
Purchase and retirement of common stock	(41)		$ (10)	(31)		(41)
Purchase and retirement of common stock, Shares		(390)
Ending Balance at Dec. 31, 2024	$ 1,515	$ 51		$ 1,772	$ (457)	$ 1,366	$ 149
Ending Balance, Shares at Dec. 31, 2024		54,215