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, 2020	814	$ 53		989	(351)	691	123
Beginning Balance, Shares at Sep. 30, 2020		56,467
Net income (loss)	250
Total other comprehensive income (loss)	92
Ending Balance at Jun. 30, 2021	1,094	$ 53		1,169	(267)	955	139
Ending Balance, Shares at Jun. 30, 2021		56,723
Beginning Balance at Dec. 31, 2020	962	$ 53		1,032	(262)	823	139
Beginning Balance, Shares at Dec. 31, 2020		56,606
Net income (loss)	85			75		75	10
Total other comprehensive income (loss)	(76)				(74)	(74)	(2)
Cash dividends paid, Common stock, per share	(20)			(20)		(20)
Cash dividends declared to noncontrolling interests	(11)						(11)
Issuance of stock under equity compensation plans	1		1			1
Issuance of stock under equity compensation plans, Shares		17
Amortization of share-based compensation	6		6			6
Purchase and retirement of common stock, Shares		(1)
Amount reclassified to retained earnings in excess of additional paid in capital			(7)	7
Ending Balance at Mar. 31, 2021	947	$ 53		1,094	(336)	811	136
Ending Balance, Shares at Mar. 31, 2021		56,622
Net income (loss)	95			86		86	9
Total other comprehensive income (loss)	72				69	69	3
Cash dividends paid, Common stock, per share	(20)			(20)		(20)
Cash dividends declared to noncontrolling interests	(9)						(9)
Issuance of stock under equity compensation plans	3		3			3
Issuance of stock under equity compensation plans, Shares		103
Amortization of share-based compensation	6		6			6
Purchase and retirement of common stock, Shares		(2)
Amount reclassified to retained earnings in excess of additional paid in capital			(9)	9
Ending Balance at Jun. 30, 2021	1,094	$ 53		1,169	(267)	955	139
Ending Balance, Shares at Jun. 30, 2021		56,723
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
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	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
Amortization of 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
Amortization of 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