Derivatives - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended
	Jun. 30, 2013	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2014 Forward Foreign Currency Contracts [Member]	Jun. 30, 2013 Forward Foreign Currency Contracts [Member]	Jun. 30, 2014 Forward Foreign Currency Contracts [Member]	Jun. 30, 2013 Forward Foreign Currency Contracts [Member]	Jun. 30, 2013 Currency Swaps [Member]	Jun. 30, 2013 Maximum [Member] Currency Swaps [Member]	Jun. 30, 2014 Fair Value Hedging [Member] Interest Rate Swap-Fixed to Variable [Member]	Sep. 30, 2013 Fair Value Hedging [Member] Interest Rate Swap-Fixed to Variable [Member]
Derivative [Line Items]
Derivatives held to manage interest rate risk										$ 0	$ 0
Derivatives designated as hedges			0
Gain (loss) on foreign currency derivative instruments not designated as hedging instruments				(1)	(2)	(5)	7	(2)	(1)
Gain (loss) on foreign currency derivatives recorded in earnings	3	1
Bond held by one of Cabot's European subsidiaries	$ 175	$ 175