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, 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