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Restructuring - Recorded Restructuring Activities (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Cost of Sales [Member] |  |  |  |
Restructuring Cost And Reserve [Line Items] |  |  |  |
Restructuring reserve, period expense | $ 1 | $ 1 | $ (8) | $ 2
Selling and Administrative Expenses [Member] |  |  |  |
Restructuring Cost And Reserve [Line Items] |  |  |  |
Restructuring reserve, period expense |  |  | 1 | 1
Research and Development Expenses [Member] |  |  |  |
Restructuring Cost And Reserve [Line Items] |  |  |  |
Restructuring reserve, period expense | $ 1 | $ 1 | $ (7) | $ 3

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

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- 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
Name: | us-gaap_RestructuringCostAndReserveLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_IncomeStatementLocationAxis=us-gaap_CostOfSalesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_IncomeStatementLocationAxis=cbt_SellingAndAdministrativeExpensesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_IncomeStatementLocationAxis=us-gaap_ResearchAndDevelopmentExpenseMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |