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Derivatives - Additional Information (Detail) - USD ($)	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Derivative [Line Items]
Significant concentration of credit risk associated with our derivative instruments	$ 0	$ 0
Maximum [Member]
Derivative [Line Items]
Fair value of derivative instruments not designated as hedges, liability	1,000,000	1,000,000
Net Investment Hedge [Member] | Cross Currency Swaps [Member]
Derivative [Line Items]
Net cash interest received	4,000,000	4,000,000	$ 4,000,000
Fair value of swaps, net asset	1,000,000	12,000,000
Cumulative gain related to swaps	$ 5,000,000	$ 15,000,000
Net Investment Hedge [Member] | Cross Currency Swaps [Member] | Fixed Rate Debt [Member]
Derivative [Line Items]
Derivative Instruments Frequency of Cash Settlements	semi-annually

X

- Definition

Accumulated other comprehensive income (loss) cumulative changes in net gain (loss) from net investment hedges net of tax.

+ References

No definition available.

+ Details

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Namespace Prefix:	cbt_
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Balance Type:	credit
Period Type:	instant

X

- Definition

Concentration risk credit risk value.

+ References

No definition available.

+ Details

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Namespace Prefix:	cbt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Derivative instruments frequency of cash settlements.

+ References

No definition available.

+ Details

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Namespace Prefix:	cbt_
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Balance Type:	na
Period Type:	duration

X

- Definition

The amount of cash received during the period on settlement of a hedge.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as a liability.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (c)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4B

+ Details

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Period Type:	instant

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480237/815-40-50-5

+ Details

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X

- Definition

Fair value of all derivative assets designated as hedges of net investment in foreign operations.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4B
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4B

+ Details

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- Details

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X

- Details

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X

- Details

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Period Type:

X

- Details

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