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, 2021	$ 1,090	$ 53	$ 24	$ 1,159	$ (289)	$ 947	$ 143
Beginning Balance, Shares at Sep. 30, 2021		56,727
Net income (loss)	(80)			(89)		(89)	9
Total other comprehensive income (loss)	(28)				(30)	(30)	2
Cash dividends paid, Common stock, per share	(21)			(21)		(21)
Issuance of stock under equity compensation plans, Shares		202
Share-based compensation	6		6			6
Purchase and retirement of common stock	(19)		(19)			(19)
Purchase and retirement of common stock, Shares		(349)
Ending Balance at Dec. 31, 2021	948	$ 53	11	1,049	(319)	794	154
Ending Balance, Shares at Dec. 31, 2021		56,580
Beginning Balance at Sep. 30, 2021	1,090	$ 53	24	1,159	(289)	947	143
Beginning Balance, Shares at Sep. 30, 2021		56,727
Net income (loss)	140
Total other comprehensive income (loss)	(85)
Ending Balance at Jun. 30, 2022	1,043	$ 53		1,210	(368)	895	148
Ending Balance, Shares at Jun. 30, 2022		56,299
Beginning Balance at Dec. 31, 2021	948	$ 53	11	1,049	(319)	794	154
Beginning Balance, Shares at Dec. 31, 2021		56,580
Net income (loss)	114			107		107	7
Total other comprehensive income (loss)	46				45		1
Cash dividends paid, Common stock, per share	(21)			(21)		(21)
Cash dividends declared to noncontrolling interests	(14)						(14)
Issuance of stock under equity compensation plans	3		3			3
Issuance of stock under equity compensation plans, Shares		72
Share-based compensation	5		5			5
Purchase and retirement of common stock	(15)		(15)			(15)
Purchase and retirement of common stock, Shares		(221)
Ending Balance at Mar. 31, 2022	1,066	$ 53	4	1,135	(274)	918	148
Ending Balance, Shares at Mar. 31, 2022		56,431
Net income (loss)	106			97		97	9
Total other comprehensive income (loss)	(103)				(94)	(94)	(9)
Cash dividends paid, Common stock, per share	(21)			(21)		(21)
Issuance of stock under equity compensation plans	3		3			3
Issuance of stock under equity compensation plans, Shares		66
Share-based compensation	6		6			6
Purchase and retirement of common stock	(14)		(13)	(1)		(14)
Purchase and retirement of common stock, Shares		(198)
Ending Balance at Jun. 30, 2022	1,043	$ 53		1,210	(368)	895	148
Ending Balance, Shares at Jun. 30, 2022		56,299
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)	238
Total other comprehensive income (loss)	113
Ending Balance at Jun. 30, 2023	1,266	$ 53		1,407	(329)	1,131	135
Ending Balance, Shares at Jun. 30, 2023		55,937
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
Net income (loss)	90			82		82	8
Total other comprehensive income (loss)	(21)				(15)	(15)	(6)
Cash dividends paid, Common stock, per share	(23)			(23)		(23)
Issuance of stock under equity compensation plans, Shares		4
Share-based compensation	3		3			3
Purchase and retirement of common stock	(15)		$ (3)	(12)		(15)
Purchase and retirement of common stock, Shares		(213)
Ending Balance at Jun. 30, 2023	$ 1,266	$ 53		$ 1,407	$ (329)	$ 1,131	$ 135
Ending Balance, Shares at Jun. 30, 2023		55,937