Consolidated Statements of Operations - USD ($) shares in Millions, $ in Millions	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Net sales and other operating revenues	$ 3,337	$ 3,242	$ 2,717
Cost of sales	2,652	2,470	2,060
Gross profit	685	772	657
Selling and administrative expenses	290	308	262
Research and technical expenses	60	66	57
Specialty Fluids loss on sale and asset impairment charge (Note D)	29
Purification Solutions long-lived assets impairment charge (Note G)		162
Purification Solutions goodwill impairment charge (Note G)		92
Income (loss) from operations	306	144	338
Interest and dividend income	9	10	9
Interest expense	(59)	(54)	(53)
Other income (expense)	(1)	17	5
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies	255	117	299
(Provision) benefit for income taxes	(70)	(193)	(33)
Equity in earnings of affiliated companies, net of tax	1	2	7
Net income (loss)	186	(74)	273
Net income (loss) attributable to noncontrolling interests, net of tax of $6, $10 and $6	29	39	25
Net income (loss) attributable to Cabot Corporation	$ 157	$ (113)	$ 248
Weighted-average common shares outstanding:
Basic	58.7	61.7	62.3
Diluted	58.8	61.7	62.7
Earnings per common share:
Basic	$ 2.64	$ (1.85)	$ 3.94
Diluted	$ 2.63	$ (1.85)	$ 3.91

Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

Name:	us-gaap_AssetImpairmentCharges
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The aggregate costs related to goods produced and sold and services rendered by an entity during the reporting period. This excludes costs incurred during the reporting period related to financial services rendered and other revenue generating activities.