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, 2022	$ 1,032	$ 52	$ 1	$ 1,284	$ (439)	$ 898	$ 134
Beginning Balance, Shares at Sep. 30, 2022		56,249
Net income (loss)	484			445		445	39
Total other comprehensive income (loss)	77				77	77
Cash dividends declared to noncontrolling interests	(30)						(30)
Cash dividends declared, Common stock, per share	(88)			(88)		(88)
Issuance of stock under equity compensation plans	4	$ 1	3			4
Issuance of stock under equity compensation plans, Shares		357
Share-based compensation	26		26			26
Purchase and retirement of common stock	(98)	$ (1)	(30)	(67)		(98)
Purchase and retirement of common stock, Shares		(1,362)
Ending Balance at Sep. 30, 2023	1,407	$ 52		1,574	(362)	1,264	143
Ending Balance, Shares at Sep. 30, 2023		55,244
Net income (loss)	424			380		380	44
Total other comprehensive income (loss)	7				2	2	5
Cash dividends declared to noncontrolling interests	(27)						(27)
Cash dividends declared, Common stock, per share	(93)			(93)		(93)
Issuance of stock under equity compensation plans	20	$ 1	19			20
Issuance of stock under equity compensation plans, Shares		912
Share-based compensation	25		25			25
Purchase and retirement of common stock	(173)	$ (2)	(44)	(127)		(173)
Purchase and retirement of common stock, Shares		(1,859)
Ending Balance at Sep. 30, 2024	1,590	$ 51	0	1,734	(360)	1,425	165
Ending Balance, Shares at Sep. 30, 2024		54,297
Net income (loss)	376			331		331	45
Total other comprehensive income (loss)	27				25	25	2
Cash dividends declared to noncontrolling interests	(57)						(57)
Cash dividends declared, Common stock, per share	(96)			(96)		(96)
Issuance of stock under equity compensation plans	8	$ 1	7			8
Issuance of stock under equity compensation plans, Shares		502
Share-based compensation	24		24			24
Purchase and retirement of common stock	(167)	$ (2)	$ (31)	(134)		(167)
Purchase and retirement of common stock, Shares		(1,957)
Ending Balance at Sep. 30, 2025	$ 1,705	$ 50		$ 1,835	$ (335)	$ 1,550	$ 155
Ending Balance, Shares at Sep. 30, 2025		52,842