Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Net sales and other operating revenues	$ 845	$ 854	$ 2,510	$ 2,392
Cost of sales	675	657	1,996	1,831
Gross profit	170	197	514	561
Selling and administrative expenses	65	74	208	223
Research and technical expenses	16	17	47	48
Specialty Fluids loss on sale and asset impairment (Note D)	8		28
Purification Solutions long-lived assets impairment (Note F)				162
Purification Solutions goodwill impairment (Note F)				92
Income (loss) from operations	81	106	231	36
Interest and dividend income	2	2	6	8
Interest expense	(14)	(14)	(43)	(41)
Other income (expense)		1	(6)	13
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies	69	95	188	16
(Provision) benefit for income taxes	(30)	4	(43)	(194)
Equity in earnings of affiliated companies, net of tax	1		1	2
Net income (loss)	40	99	146	(176)
Net income (loss) attributable to noncontrolling interests, net of tax	8	11	22	31
Net income (loss) attributable to Cabot Corporation	$ 32	$ 88	$ 124	$ (207)
Weighted-average common shares outstanding:
Basic	58.2	61.8	59.1	61.8
Diluted	58.4	62.3	59.2	61.8
Earnings per common share:
Basic	$ 0.55	$ 1.41	$ 2.08	$ (3.36)
Diluted	0.55	1.40	2.08	(3.36)
Dividends per common share	$ 0.35	$ 0.33	$ 1.01	$ 0.96