Derivatives (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Derivative [Line Items]
Gain (loss) on foreign currency derivatives recorded in earnings	$ 3	$ 12	$ 1	$ 17
Bond held by one of Cabot's European subsidiaries	175	175	175	175
Maximum [Member]
Derivative [Line Items]
Gain (loss) on foreign currency derivative instruments not designated as hedging instruments	1		1
Forward Foreign Currency Contracts [Member]
Derivative [Line Items]
Gain (loss) on foreign currency derivative instruments not designated as hedging instruments	2	6	7	3
Currency Swap [Member]
Derivative [Line Items]
Gain (loss) on foreign currency derivative instruments not designated as hedging instruments	$ 2	$ 13		$ 18