Accumulated Other Comprehensive Income - Changes in Each Component of Accumulated Other Comprehensive Income, Net of Tax (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at September 30, 2013, attributable to Cabot Corporation			$ 103
Other comprehensive loss before reclassifications			(18)
Amounts reclassified from accumulated other comprehensive income			3
Other comprehensive income (loss)	13	(2)	(15)	(21)
Less: Noncontrolling interest			(2)
Balance at June 30, 2014, attributable to Cabot Corporation	90		90
Currency Translation Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at September 30, 2013, attributable to Cabot Corporation			154
Other comprehensive loss before reclassifications			(18)
Amounts reclassified from accumulated other comprehensive income
Other comprehensive income (loss)			(18)
Less: Noncontrolling interest			(2)
Balance at June 30, 2014, attributable to Cabot Corporation	138		138
Unrealized Gains on Investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at September 30, 2013, attributable to Cabot Corporation			2
Other comprehensive loss before reclassifications
Amounts reclassified from accumulated other comprehensive income			2
Other comprehensive income (loss)			2
Less: Noncontrolling interest
Balance at June 30, 2014, attributable to Cabot Corporation	4		4
Pension and Other Postretirement Benefit Liability Adjustments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at September 30, 2013, attributable to Cabot Corporation			(53)
Other comprehensive loss before reclassifications
Amounts reclassified from accumulated other comprehensive income			1
Other comprehensive income (loss)			1
Less: Noncontrolling interest
Balance at June 30, 2014, attributable to Cabot Corporation	$ (52)		$ (52)