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Significant Accounting Policies - Segment Reporting Revenue Percentage (Detail)
 3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Reinforcement Materials [Member]    
Segment Reporting Information [Line Items]    
Revenue, by segment 59.00%cbt_SegmentReportingInformationRevenues
/ us-gaap_StatementBusinessSegmentsAxis
= cbt_ReinforcementMaterialsMember		 62.00%cbt_SegmentReportingInformationRevenues
/ us-gaap_StatementBusinessSegmentsAxis
= cbt_ReinforcementMaterialsMember
Performance Chemicals [Member]    
Segment Reporting Information [Line Items]    
Revenue, by segment 29.00%cbt_SegmentReportingInformationRevenues
/ us-gaap_StatementBusinessSegmentsAxis
= cbt_PerformanceChemicalsMember		 27.00%cbt_SegmentReportingInformationRevenues
/ us-gaap_StatementBusinessSegmentsAxis
= cbt_PerformanceChemicalsMember
Purification Solutions [Member]    
Segment Reporting Information [Line Items]    
Revenue, by segment 10.00%cbt_SegmentReportingInformationRevenues
/ us-gaap_StatementBusinessSegmentsAxis
= cbt_PurificationSolutionsMember		 8.00%cbt_SegmentReportingInformationRevenues
/ us-gaap_StatementBusinessSegmentsAxis
= cbt_PurificationSolutionsMember
Specialty Fluids [Member]    
Segment Reporting Information [Line Items]    
Revenue, by segment 2.00%cbt_SegmentReportingInformationRevenues
/ us-gaap_StatementBusinessSegmentsAxis
= cbt_SpecialtyFluidsSegmentMember		 3.00%cbt_SegmentReportingInformationRevenues
/ us-gaap_StatementBusinessSegmentsAxis
= cbt_SpecialtyFluidsSegmentMember