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Income Taxes - Expiration Dates of NOLs, Capital Loss and Other Tax Credit Carryforwards Before Valuation Allowances (Parenthetical) (Detail)	12 Months Ended
	Sep. 30, 2025
Significant Tax Attributes And Dates Of Expiration [Line Items]
Expiration periods	2033
Minimum [Member]
Significant Tax Attributes And Dates Of Expiration [Line Items]
Expiration periods	2026
Maximum [Member]
Significant Tax Attributes And Dates Of Expiration [Line Items]
Expiration periods	2032

X

- Definition

Significant tax attributes and dates of expiration.

+ References

No definition available.

+ Details

Name:	cbt_SignificantTaxAttributesAndDatesOfExpirationLineItems
Namespace Prefix:	cbt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Tax credit carryforward expiration period.

+ References

No definition available.

+ Details

Name:	cbt_TaxCreditCarryforwardExpirationPeriod
Namespace Prefix:	cbt_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: