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Financial Information by Segment & Geographic Area - Schedule of Details of Certain Items (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Segment Reporting Information [Line Items] |  |  |
Indirect tax settlement charges | $ (7) |  |
Loss on sale of business |  |  | $ (3)
Certain Items [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Global restructuring activities | (11) | $ (13) | (4)
Indirect tax settlement charges | (7) |  |
Legal and environmental matters and reserves | (7) | (2) | (10)
Employee benefit plan settlement and other charges | (3) |  |
Acquisition and integration-related charges | (1) |  | (4)
Loss on sale of business |  |  | (3)
Other certain items | (1) | (1) | (1)
Total certain items | $ (30) | (59) | (29)
Certain Items [Member] | Argentina [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Currency devaluation and other losses |  | $ (43) | $ (7)

X
- Definition Amount excluded from earnings before interest and tax. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Business combination acquisition and integration related costs. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Currency devaluation other losses. + References No definition available. + Details
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Balance Type: | debit
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X
- Definition Employee benefit plan settlement and other charges. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Indirect tax settlement charges. + References No definition available. + Details
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Period Type: | duration

X
- Definition Legal environmental matters and reserves. + References No definition available. + Details
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Period Type: | duration

X
- Definition Other certain items. + References No definition available. + Details
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Balance Type: | debit
Period Type: | duration

X
- Definition Restructuring charges benefit. + References No definition available. + Details
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Namespace Prefix: | cbt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of gain (loss) from sale and disposal of integrated set of activities and assets capable of being conducted and managed for purpose of providing return in form of dividend, lower cost, or other economic benefit to investor, owner, member and participant. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 230 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 28 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482740/230-10-45-28 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 810 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1B -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481203/810-10-50-1B + Details
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X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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X
- Details
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X
- Details
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