Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Net sales and other operating revenues	$ 844	$ 818	$ 1,665	$ 1,538
Cost of sales	666	630	1,321	1,174
Gross profit	178	188	344	364
Selling and administrative expenses	70	79	143	149
Research and technical expenses	15	16	31	31
Specialty Fluids held for sale assets impairment (Note D)	20		20
Purification Solutions long-lived assets impairment (Note F)		162		162
Purification Solutions goodwill impairment (Note F)		92		92
Income (loss) from operations	73	(161)	150	(70)
Interest and dividend income	2	3	4	6
Interest expense	(14)	(14)	(29)	(27)
Other income (expense)	(12)	1	(6)	12
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies	49	(171)	119	(79)
(Provision) benefit for income taxes	(20)	7	(13)	(198)
Equity in earnings of affiliated companies, net of tax		1		2
Net income (loss)	29	(163)	106	(275)
Net income (loss) attributable to noncontrolling interests, net of tax	6	10	14	20
Net income (loss) attributable to Cabot Corporation	$ 23	$ (173)	$ 92	$ (295)
Weighted-average common shares outstanding:
Basic	59.1	61.8	59.5	61.8
Diluted	59.3	61.8	59.7	61.8
Earnings per common share:
Basic	$ 0.39	$ (2.80)	$ 1.53	$ (4.78)
Diluted	0.39	(2.80)	1.53	(4.78)
Dividends per common share	$ 0.33	$ 0.315	$ 0.66	$ 0.63