Derivatives - Additional Information (Detail) - USD ($)	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2015	Sep. 30, 2014
Derivative [Line Items]
Significant concentration of credit risk associated with our derivative instruments		$ 0
Derivatives designated as hedges		0	$ 0
Gain on foreign currency derivatives recorded in earnings	$ 4,000,000
Bond held by one of Cabot's European subsidiaries	175,000,000
Forward Foreign Currency Contracts [Member]
Derivative [Line Items]
Gain (loss) on foreign currency derivative instruments not designated as hedging instruments	5,000,000
Currency Swaps [Member]
Derivative [Line Items]
Gain (loss) on foreign currency derivative instruments not designated as hedging instruments	$ (2,000,000)
Fair Value Hedging [Member] | Interest Rate Swap-Fixed to Variable [Member]
Derivative [Line Items]
Derivatives held to manage interest rate risk		$ 0	$ 0