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, 2022	1,032	$ 52	1	1,284	(439)	898	134
Beginning Balance, Shares at Sep. 30, 2022		56,249
Net income (loss)	148
Total other comprehensive income (loss)	134
Ending Balance at Mar. 31, 2023	1,232	$ 53		1,360	(314)	1,099	133
Ending Balance, Shares at Mar. 31, 2023		56,146
Beginning Balance at Dec. 31, 2022	1,159	$ 53		1,314	(358)	1,009	150
Beginning Balance, Shares at Dec. 31, 2022		56,316
Net income (loss)	82			75		75	7
Total other comprehensive income (loss)	47				44	44	3
Cash dividends paid, Common stock, per share	(21)			(21)		(21)
Cash dividends declared to noncontrolling interests	(27)						(27)
Issuance of stock under equity compensation plans	1		1			1
Issuance of stock under equity compensation plans, Shares		29
Share-based compensation	7		7			7
Purchase and retirement of common stock	(16)		(8)	(8)		(16)
Purchase and retirement of common stock, Shares		(199)
Ending Balance at Mar. 31, 2023	1,232	$ 53		1,360	(314)	1,099	133
Ending Balance, Shares at Mar. 31, 2023		56,146
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
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)	158
Total other comprehensive income (loss)	45
Ending Balance at Mar. 31, 2024	1,507	$ 52		1,632	(317)	1,367	140
Ending Balance, Shares at Mar. 31, 2024		55,258
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