Consolidated Statements of Operations - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Net sales and other operating revenues	$ 2,614	$ 3,337	$ 3,242
Cost of sales	2,114	2,652	2,470
Gross profit	500	685	772
Selling and administrative expenses	292	290	308
Research and technical expenses	57	60	66
Purification Solutions long-lived assets impairment charge (Note G)			162
Purification Solutions goodwill impairment charge (Note G)			92
Income (loss) from operations	21	306	144
Interest and dividend income	8	9	10
Interest expense	(53)	(59)	(54)
Other income (expense)	(9)	(1)	17
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies	(33)	255	117
(Provision) benefit for income taxes	(191)	(70)	(193)
Equity in earnings of affiliated companies, net of tax	3	1	2
Net income (loss)	(221)	186	(74)
Net income (loss) attributable to noncontrolling interests, net of tax of $4, $6 and $10	17	29	39
Net income (loss) attributable to Cabot Corporation	$ (238)	$ 157	$ (113)
Weighted-average common shares outstanding:
Basic	56.6	58.7	61.7
Diluted	56.6	58.8	61.7
Earnings per common share:
Basic	$ (4.21)	$ 2.64	$ (1.85)
Diluted	$ (4.21)	$ 2.63	$ (1.85)
Specialty Fluids [Member]
Loss on sale and asset impairment charge	$ 1	$ 29
Marshall Mine [Member]
Loss on sale and asset impairment charge	$ 129