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, 2023	1,407	$ 52	0	1,574	(362)	1,264	143
Beginning Balance, Shares at Sep. 30, 2023		55,244
Net income (loss)	278
Total other comprehensive income (loss)	(21)
Ending Balance at Jun. 30, 2024	1,496	$ 52		1,676	(382)	1,346	150
Ending Balance, Shares at Jun. 30, 2024		54,817
Beginning Balance at Dec. 31, 2023	1,475	$ 52		1,582	(305)	1,329	146
Beginning Balance, Shares at Dec. 31, 2023		55,425
Net income (loss)	97			84		84	13
Total other comprehensive income (loss)	(16)				(12)	(12)	(4)
Cash dividends paid, Common stock, per share	(23)			(23)		(23)
Cash dividends declared to noncontrolling interests	(15)						(15)
Issuance of stock under equity compensation plans	6		6			6
Issuance of stock under equity compensation plans, Shares		122
Share-based compensation	7		7			7
Purchase and retirement of common stock	(24)		(13)	(11)		(24)
Purchase and retirement of common stock, Shares		(289)
Ending Balance at Mar. 31, 2024	1,507	$ 52		1,632	(317)	1,367	140
Ending Balance, Shares at Mar. 31, 2024		55,258
Net income (loss)	120			109		109	11
Total other comprehensive income (loss)	(66)				(65)	(65)	(1)
Cash dividends paid, Common stock, per share	(24)			(24)		(24)
Issuance of stock under equity compensation plans	2	$ 1	1			2
Issuance of stock under equity compensation plans, Shares		57
Share-based compensation	6		6			6
Purchase and retirement of common stock	(49)	$ (1)	(7)	(41)		(49)
Purchase and retirement of common stock, Shares		(498)
Ending Balance at Jun. 30, 2024	1,496	$ 52		1,676	(382)	1,346	150
Ending Balance, Shares at Jun. 30, 2024		54,817
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
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)	322
Total other comprehensive income (loss)	5
Ending Balance at Jun. 30, 2025	1,683	$ 50		1,846	(355)	1,541	142
Ending Balance, Shares at Jun. 30, 2025		53,203
Beginning Balance at Dec. 31, 2024	1,515	$ 51		1,772	(457)	1,366	149
Beginning Balance, Shares at Dec. 31, 2024		54,215
Net income (loss)	105			94		94	11
Total other comprehensive income (loss)	33				30	30	3
Cash dividends paid, Common stock, per share	(23)			(23)		(23)
Issuance of stock under equity compensation plans, Shares		16
Share-based compensation	6		6			6
Purchase and retirement of common stock	(46)		(6)	(40)		(46)
Purchase and retirement of common stock, Shares		(530)
Ending Balance at Mar. 31, 2025	1,590	$ 51		1,803	(427)	1,427	163
Ending Balance, Shares at Mar. 31, 2025		53,701
Net income (loss)	113			101		101	12
Total other comprehensive income (loss)	76				72	72	4
Cash dividends paid, Common stock, per share	(24)			(24)		(24)
Cash dividends declared to noncontrolling interests	(37)						(37)
Issuance of stock under equity compensation plans, Shares		38
Share-based compensation	5		5			5
Purchase and retirement of common stock	(40)	$ (1)	$ (5)	(34)		(40)
Purchase and retirement of common stock, Shares		(536)
Ending Balance at Jun. 30, 2025	$ 1,683	$ 50		$ 1,846	$ (355)	$ 1,541	$ 142
Ending Balance, Shares at Jun. 30, 2025		53,203