Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Net sales and other operating revenues	$ 518	$ 845	$ 1,955	$ 2,510
Cost of sales	449	675	1,592	1,996
Gross profit	69	170	363	514
Selling and administrative expenses	52	65	230	208
Research and technical expenses	13	16	41	47
Specialty Fluids loss on sale and asset impairment		8	1	28
Income (loss) from operations	4	81	91	231
Interest and dividend income	1	2	7	6
Interest expense	(13)	(14)	(41)	(43)
Other income (expense)	(3)		(6)	(6)
Income (loss) before income taxes and equity in earnings of affiliated companies	(11)	69	51	188
(Provision) benefit for income taxes	5	(30)	(9)	(43)
Equity in earnings of affiliated companies, net of tax	1	1	2	1
Net income (loss)	(5)	40	44	146
Net income (loss) attributable to noncontrolling interests, net of tax	1	8	10	22
Net income (loss) attributable to Cabot Corporation	$ (6)	$ 32	$ 34	$ 124
Weighted-average common shares outstanding:
Basic	56.5	58.2	56.7	59.1
Diluted	56.5	58.4	56.7	59.2
Earnings (loss) per common share:
Basic	$ (0.12)	$ 0.55	$ 0.59	$ 2.08
Diluted	$ (0.12)	$ 0.55	$ 0.59	$ 2.08