Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
Net sales and other operating revenues	$ 842	$ 710	$ 1,588	$ 1,437
Cost of sales	628	557	1,181	1,143
Gross profit	214	153	407	294
Selling and administrative expenses	71	114	132	178
Research and technical expenses	15	14	29	28
Specialty Fluids loss on sale and asset impairment charge		1		1
Income (loss) from operations	128	24	246	87
Interest and dividend income	2	3	4	6
Interest expense	(13)	(14)	(25)	(28)
Other income (expense)	1	(1)	(8)	(3)
Income (loss) before income taxes and equity in earnings of affiliated companies	118	12	217	62
(Provision) benefit for income taxes	(34)	(10)	(63)	(14)
Equity in earnings of affiliated companies, net of tax	1	1	1	1
Net income (loss)	85	3	155	49
Net income (loss) attributable to noncontrolling interests, net of tax	10	4	20	9
Net income (loss) attributable to Cabot Corporation	$ 75	$ (1)	$ 135	$ 40
Weighted-average common shares outstanding:
Basic	56.6	56.6	56.6	56.7
Diluted	56.7	56.6	56.7	56.8
Earnings (loss) per common share:
Basic	$ 1.31	$ (0.01)	$ 2.37	$ 0.70
Diluted	$ 1.30	$ (0.01)	$ 2.36	$ 0.70