Consolidated Statements of Comprehensive (Loss) Income (USD $) In Millions, unless otherwise specified	3 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Net income	$ 49us-gaap_ProfitLoss	$ 86us-gaap_ProfitLoss
Other comprehensive loss, net of tax
Foreign currency translation adjustment (net of tax provision of $1 and $-)	(103)	(21)
Pension and other postretirement benefit liability adjustments
Pension and other postretirement benefit liability adjustments arising during the period (net of tax benefit of $6 and $-)	21
Other comprehensive loss	(82)	(21)
Comprehensive (loss) income	(33)	65
Net income attributable to noncontrolling interests	4	6
Noncontrolling interests foreign currency translation adjustment, net of tax	(2)	(1)
Comprehensive income attributable to noncontrolling interests, net of tax	2	5
Comprehensive (loss) income attributable to Cabot Corporation	$ (35)us-gaap_ComprehensiveIncomeNetOfTax	$ 60us-gaap_ComprehensiveIncomeNetOfTax

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Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income (loss) and other comprehensive income (loss), attributable to noncontrolling interests. Excludes changes in equity resulting from investments by owners and distributions to owners.

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Amount of Net Income (Loss) attributable to noncontrolling interest.

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Amount after tax and reclassification adjustments of gain (loss) on foreign currency translation adjustments, foreign currency transactions designated and effective as economic hedges of a net investment in a foreign entity and intra-entity foreign currency transactions that are of a long-term-investment nature, attributable to noncontrolling interests.

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Amount after tax and reclassification adjustments of other comprehensive income (loss).

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The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

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