Financial Information by Segment (Narrative) (Detail) (Subsequent Event [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
Sep. 30, 2013
Subsequent Event [Member]
Segment Reporting Information [Line Items]
Expected allocation of functional costs	$ 12

Maximum expected functional cost allocation for year ended to different segment.

No definition available.

Name:	cbt_MaximumExpectedFunctionalCostAllocationForYearEndedToDifferentSegment
Namespace Prefix:	cbt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration