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
	Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2013 Currency Translation Adjustment [Member]		Dec. 31, 2013 Unrealized Gains on Investments [Member]	Sep. 30, 2013 Unrealized Gains on Investments [Member]	Dec. 31, 2013 Pension and Other Postretirement Benefit Liability Adjustment [Member]	Sep. 30, 2013 Pension and Other Postretirement Benefit Liability Adjustment [Member]
Components Of Accumulated Other Comprehensive Income Loss [Line Items]
Balance at September 30, 2013	$ 103			$ 154		$ 2	$ 2	$ (53)	$ (53)
Other comprehensive loss before reclassifications	(21)			(21)
Net other comprehensive items	(21)	[1]	16	(21)	[1]
Balance at December 31, 2013	83			133		2	2	(53)	(53)
Noncontrolling interest	(1)			(1)
Balance at December 31, 2013, attributable to Cabot Corporation	$ 83			$ 134		$ 2		$ (53)

[1] The amounts reclassified during the three months ended December 31, 2013 were less than $1 million.