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, 2019	$ 1,134	$ 52		$ 1,337	$ (391)	$ 998	$ 136
Beginning Balance, Shares at Sep. 30, 2019		57,081
Adoption of accounting standards				3	(3)
Net income (loss)	46			41		41	5
Total other comprehensive income (loss)	43				40	40	3
Cash dividends paid, Common stock, per share	(20)			(20)		(20)
Cash dividends declared to noncontrolling interests	(19)						(19)
Issuance of stock under equity compensation plans	1		$ 1			1
Issuance of stock under equity compensation plans, Shares		273
Amortization of share-based compensation	1		1			1
Purchase and retirement of common stock	(34)		(2)	(32)		(34)
Purchase and retirement of common stock, Shares		(699)
Ending Balance at Dec. 31, 2019	1,152	$ 52		1,329	(354)	1,027	125
Ending Balance, Shares at Dec. 31, 2019		56,655
Beginning Balance at Sep. 30, 2019	1,134	$ 52		1,337	(391)	998	136
Beginning Balance, Shares at Sep. 30, 2019		57,081
Net income (loss)	44
Total other comprehensive income (loss)	(4)
Ending Balance at Jun. 30, 2020	1,043	$ 53		1,277	(400)	930	113
Ending Balance, Shares at Jun. 30, 2020		56,460
Beginning Balance at Dec. 31, 2019	1,152	$ 52		1,329	(354)	1,027	125
Beginning Balance, Shares at Dec. 31, 2019		56,655
Net income (loss)	3			(1)		(1)	4
Total other comprehensive income (loss)	(78)				(75)	(75)	(3)
Cash dividends paid, Common stock, per share	(20)			(20)		(20)
Cash dividends declared to noncontrolling interests	(16)						(16)
Issuance of stock under equity compensation plans	2		2			2
Issuance of stock under equity compensation plans, Shares		30
Amortization of share-based compensation	2		2			2
Purchase and retirement of common stock	(10)		(4)	(6)		(10)
Purchase and retirement of common stock, Shares		(241)
Ending Balance at Mar. 31, 2020	1,035	$ 52		1,302	(429)	925	110
Ending Balance, Shares at Mar. 31, 2020		56,444
Net income (loss)	(5)			(6)		(6)	1
Total other comprehensive income (loss)	31				29	29	2
Cash dividends paid, Common stock, per share	(20)			(20)		(20)
Issuance of stock under equity compensation plans		$ 1	(1)
Issuance of stock under equity compensation plans, Shares		18
Amortization of share-based compensation	2		2			2
Purchase and retirement of common stock			(1)	1
Purchase and retirement of common stock, Shares		(2)
Ending Balance at Jun. 30, 2020	1,043	$ 53		1,277	(400)	930	113
Ending Balance, Shares at Jun. 30, 2020		56,460
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