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Financial Information by Segment - Financial Information by Reportable Segment (Parenthetical) (Detail) | 3 Months Ended | 9 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Segment Reporting [Abstract] |  |  |  |
Percentage of sale of an equity method affiliate | 100.00% | 100.00% | 100.00% | 100.00%

X
- Definition Percentage of sale of equity method affiliates. + References No definition available. + Details
Name: | cbt_PercentageOfSaleOfEquityMethodAffiliates
Namespace Prefix: | cbt_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_SegmentReportingAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration