Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Statement [Abstract]
Net sales and other operating revenues	$ 917	$ 518	$ 2,505	$ 1,955
Cost of sales	703	449	1,884	1,592
Gross profit	214	69	621	363
Selling and administrative expenses	68	52	200	230
Research and technical expenses	12	13	41	41
Specialty Fluids loss on sale and asset impairment charge				1
Income (loss) from operations	134	4	380	91
Interest and dividend income	2	1	6	7
Interest expense	(12)	(13)	(37)	(41)
Other income (expense)	(1)	(3)	(9)	(6)
Income (loss) before income taxes and equity in earnings of affiliated companies	123	(11)	340	51
(Provision) benefit for income taxes	(30)	5	(93)	(9)
Equity in earnings of affiliated companies, net of tax	2	1	3	2
Net income (loss)	95	(5)	250	44
Net income (loss) attributable to noncontrolling interests, net of tax	9	1	29	10
Net income (loss) attributable to Cabot Corporation	$ 86	$ (6)	$ 221	$ 34
Weighted-average common shares outstanding:
Basic	56.7	56.5	56.7	56.7
Diluted	57.0	56.5	56.8	56.7
Earnings (loss) per common share:
Basic	$ 1.48	$ (0.12)	$ 3.85	$ 0.59
Diluted	$ 1.48	$ (0.12)	$ 3.84	$ 0.59