Derivatives (Foreign Currency Risk Management) (Detail) In Millions, unless otherwise specified	Jun. 30, 2013 Eurobond 80% [Member] USD ($)	Sep. 30, 2012 Eurobond 80% [Member]	Jun. 30, 2013 Eurobond 20% [Member] USD ($)	Sep. 30, 2012 Eurobond 20% [Member]	Jun. 30, 2013 Not Designated as Hedging Instrument [Member] Eurobond 80% [Member] USD ($)	Jun. 30, 2013 Not Designated as Hedging Instrument [Member] Eurobond 80% [Member] EUR (€)	Sep. 30, 2012 Not Designated as Hedging Instrument [Member] Eurobond 80% [Member] USD ($)	Sep. 30, 2012 Not Designated as Hedging Instrument [Member] Eurobond 80% [Member] EUR (€)	Jun. 30, 2013 Not Designated as Hedging Instrument [Member] Eurobond 20% [Member] USD ($)	Jun. 30, 2013 Not Designated as Hedging Instrument [Member] Eurobond 20% [Member] EUR (€)	Sep. 30, 2012 Not Designated as Hedging Instrument [Member] Eurobond 20% [Member] USD ($)	Sep. 30, 2012 Not Designated as Hedging Instrument [Member] Eurobond 20% [Member] EUR (€)	Jun. 30, 2013 Not Designated as Hedging Instrument [Member] Forward Foreign Currency Contracts [Member] USD ($)		Sep. 30, 2012 Not Designated as Hedging Instrument [Member] Forward Foreign Currency Contracts [Member] USD ($)		Jun. 30, 2013 Cash Flow Hedging [Member] Forward Foreign Currency Contracts [Member] USD ($)		Sep. 30, 2012 Cash Flow Hedging [Member] Forward Foreign Currency Contracts [Member] USD ($)
Derivative [Line Items]
Long-term debt	$ 175		$ 175
Percentage of debt hedged by cross currency swaps	80.00%	80.00%	20.00%	20.00%
Notional amount of foreign currency derivative instruments not designated as hedging instruments					$ 140	€ 124	$ 140	€ 124	$ 35	€ 31	$ 35	€ 31	$ 23	[1]	$ 106	[1]	$ 0	[1]	$ 3	[1]

[1] (1) Cabot's forward foreign exchange contracts are denominated primarily in the Australian dollar, British pound sterling, Canadian dollar, Chinese renminbi, Japanese Yen and Euro.