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, 2020	$ 814	$ 53		$ 989	$ (351)	$ 691	$ 123
Beginning Balance, Shares at Sep. 30, 2020		56,467
Net income (loss)	286			250		250	36
Total other comprehensive income (loss)	69				62	62	7
Cash dividends declared to noncontrolling interests	(23)						(23)
Cash dividends declared, Common stock, per share	(80)			(80)		(80)
Issuance of stock under equity compensation plans	6		$ 6			6
Issuance of stock under equity compensation plans, Shares		317
Stock-based compensation	21		21			21
Purchase and retirement of common stock	(3)		(3)			(3)
Purchase and retirement of common stock, Shares		(57)
Ending Balance at Sep. 30, 2021	1,090	$ 53	24	1,159	(289)	947	143
Ending Balance, Shares at Sep. 30, 2021		56,727
Net income (loss)	243			209		209	34
Total other comprehensive income (loss)	(165)				(150)	(150)	(15)
Cash dividends declared to noncontrolling interests	(28)						(28)
Cash dividends declared, Common stock, per share	(84)			(84)		84
Issuance of stock under equity compensation plans	6		6			6
Issuance of stock under equity compensation plans, Shares		359
Stock-based compensation	23		23			23
Purchase and retirement of common stock	(53)	$ (1)	(52)			(53)
Purchase and retirement of common stock, Shares		(837)
Ending Balance at Sep. 30, 2022	1,032	$ 52	1	1,284	(439)	898	134
Ending 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
Stock-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