Derivatives - Foreign Currency Risk Management (Detail)	Sep. 30, 2013 Eurobond 80% [Member] USD ($)	Sep. 30, 2012 Eurobond 80% [Member] USD ($)	Sep. 30, 2013 Eurobond 20% [Member] USD ($)	Sep. 30, 2012 Eurobond 20% [Member] USD ($)	Sep. 30, 2012 Not Designated as Hedging Instrument [Member] Eurobond 80% [Member] USD ($)	Sep. 30, 2012 Not Designated as Hedging Instrument [Member] Eurobond 80% [Member] EUR (€)	Sep. 30, 2012 Not Designated as Hedging Instrument [Member] Eurobond 20% [Member] USD ($)	Sep. 30, 2012 Not Designated as Hedging Instrument [Member] Eurobond 20% [Member] EUR (€)	Sep. 30, 2013 Not Designated as Hedging Instrument [Member] Forward Foreign Currency Contracts [Member] USD ($)		Sep. 30, 2012 Not Designated as Hedging Instrument [Member] Forward Foreign Currency Contracts [Member] USD ($)		Sep. 30, 2012 Cash Flow Hedging [Member] Forward Foreign Currency Contracts [Member] USD ($)
Derivative [Line Items]
Long-term debt	$ 175,000,000	$ 175,000,000	$ 175,000,000	$ 175,000,000
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					$ 140,000,000	€ 124,000,000	$ 35,000,000	€ 31,000,000	$ 31,000,000	[1]	$ 106,000,000	[1]	$ 3,000,000	[1]

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