Derivatives - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		3 Months Ended
	Dec. 31, 2012	Dec. 31, 2013	Dec. 31, 2012 Currency Swaps [Member]	Dec. 31, 2013 Forward Foreign Currency Contracts [Member]	Dec. 31, 2012 Forward Foreign Currency Contracts [Member]	Dec. 31, 2013 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			(4)	2	5
Gain (loss) on foreign currency derivatives recorded in earnings	4
Bond held by one of Cabot's European subsidiaries	$ 175