Financial Instruments (Foreign Currency Risk Management) (Detail) In Millions, unless otherwise specified	Sep. 30, 2012 Eurobond 80% [Member] USD ($)	Sep. 30, 2012 Eurobond 80% [Member] EUR (€)	Sep. 30, 2011 Eurobond 80% [Member] USD ($)	Sep. 30, 2011 Eurobond 80% [Member] EUR (€)	Sep. 30, 2012 Eurobond (20% of $175 million) [Member] USD ($)	Sep. 30, 2012 Eurobond (20% of $175 million) [Member] EUR (€)	Sep. 30, 2011 Eurobond (20% of $175 million) [Member] USD ($)	Sep. 30, 2011 Eurobond (20% of $175 million) [Member] EUR (€)	Sep. 30, 2012 Forward Foreign Currency Contracts [Member] USD ($)		Sep. 30, 2011 Forward Foreign Currency Contracts [Member] USD ($)
Debt Instrument [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	106	[1]	54	[1]
Notional Amount of Foreign Currency Cash Flow Hedge Derivatives									$ 3	[1]	$ 12	[1]

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