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, 2020	$ 814	$ 53		$ 989	$ (351)	$ 691	$ 123
Beginning Balance, Shares at Sep. 30, 2020		56,467
Net income (loss)	70			60		60	10
Total other comprehensive income (loss)	96				89	89	7
Cash dividends paid, Common stock, per share	(20)			(20)		(20)
Cash dividends declared to noncontrolling interests	(1)						(1)
Issuance of stock under equity compensation plans	1		$ 1			1
Issuance of stock under equity compensation plans, Shares		192
Amortization of share-based compensation	4		4			4
Purchase and retirement of common stock	(2)		(2)			(2)
Purchase and retirement of common stock, Shares		(53)
Amount reclassified to retained earnings in excess of additional paid in capital			(3)	3
Ending Balance at Dec. 31, 2020	962	$ 53		1,032	(262)	823	139
Ending Balance, Shares at Dec. 31, 2020		56,606
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
Amortization of 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