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Financial Information by Segment - Financial Information by Reportable Segment (Parenthetical) (Detail)
3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Segment Reporting [Abstract]    
Percentage of sale of an equity method affiliate 100.00%cbt_PercentageOfSaleOfEquityMethodAffiliates 100.00%cbt_PercentageOfSaleOfEquityMethodAffiliates