Consolidated Statement 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, 2017	$ 1,625	$ 56		$ 1,707	$ (259)	$ 1,504	$ 121
Beginning Balance, Shares at Sep. 30, 2017		61,884
Net income (loss)	(112)			(122)		(122)	10
Total other comprehensive income (loss)	(1)				(4)	(4)	3
Cash dividends paid to common stockholders	(20)			(20)		(20)
Cash dividends declared to noncontrolling interests	(10)						(10)
Issuance of stock under equity compensation plans, Shares		178
Amortization of share-based compensation	6		$ 6			6
Purchase and retirement of common stock	(16)		(6)	(10)		(16)
Purchase and retirement of common stock, Shares		(265)
Ending Balance at Dec. 31, 2017	1,472	$ 56		1,555	(263)	1,348	124
Ending Balance, Shares at Dec. 31, 2017		61,797
Beginning Balance at Sep. 30, 2018	1,279	$ 54		1,417	(317)	1,154	125
Beginning Balance, Shares at Sep. 30, 2018		60,367
Net income (loss)	77			69		69	8
Total other comprehensive income (loss)	(3)				(3)	(3)
Cash dividends paid to common stockholders	(20)			(20)		(20)
Issuance of stock under equity compensation plans, Shares		344
Amortization of share-based compensation	5		5			5
Purchase and retirement of common stock	(62)	$ (1)	$ (5)	(56)		(62)
Purchase and retirement of common stock, Shares		(1,201)
Ending Balance at Dec. 31, 2018	$ 1,276	$ 53		$ 1,410	$ (320)	$ 1,143	$ 133
Ending Balance, Shares at Dec. 31, 2018		59,510