Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Income Statement [Abstract]
Net sales and other operating revenues	$ 936	$ 1,019	$ 1,891	$ 1,977
Cost of sales	695	773	1,415	1,513
Gross profit	241	246	476	464
Selling and administrative expenses	64	75	130	142
Research and technical expenses	15	15	29	30
Income (loss) from operations	162	156	317	292
Interest and dividend income	7	8	13	17
Interest expense	(19)	(21)	(37)	(43)
Other income (expense)	1	(1)	2	(30)
Income (loss) from operations before income taxes and equity in earnings of affiliated companies	151	142	295	236
(Provision) benefit for income taxes	(49)	(47)	(90)	(81)
Equity in earnings of affiliated companies, net of tax	3	2	4	3
Net income (loss)	105	97	209	158
Net income (loss) attributable to noncontrolling interests, net of tax	11	13	22	24
Net income (loss) attributable to Cabot Corporation	$ 94	$ 84	$ 187	$ 134
Weighted-average common shares outstanding:
Basic	54.0	55.4	54.2	55.3
Diluted	54.4	55.8	54.7	55.8
Earnings (loss) per common share:
Basic	$ 1.71	$ 1.5	$ 3.4	$ 2.39
Diluted	$ 1.69	$ 1.49	$ 3.36	$ 2.37