Financial Instruments (Foreign Currency Risk Management) (Details)	Dec. 31, 2011 USD ($)	Dec. 31, 2011 Eurobond 20% [Member] USD ($)	Dec. 31, 2011 Eurobond 20% [Member] EUR (€)	Sep. 30, 2011 Eurobond 20% [Member] USD ($)	Sep. 30, 2011 Eurobond 20% [Member] EUR (€)	Dec. 31, 2011 Eurobond 80% [Member] USD ($)	Dec. 31, 2011 Eurobond 80% [Member] EUR (€)	Sep. 30, 2011 Eurobond 80% [Member] USD ($)	Sep. 30, 2011 Eurobond 80% [Member] EUR (€)	Dec. 31, 2011 Forward Foreign Currency Contracts [Member] USD ($)		Sep. 30, 2011 Forward Foreign Currency Contracts [Member] USD ($)
Debt Instrument [Line Items]
Long-term debt	$ 175,000,000	$ 175,000,000				$ 175,000,000
Percentage of debt hedged by cross currency swaps		20.00%	20.00%			80.00%	80.00%
Notional amount of foreign currency derivative instruments not designated as hedging instruments		35,000,000	31,000,000	35,000,000	31,000,000	140,000,000	124,000,000	140,000,000	124,000,000	211,000,000	[1]	54,000,000	[1]
Notional Amount of Foreign Currency Cash Flow Hedge Derivatives										$ 11,000,000	[2]	$ 12,000,000	[2]

[1] Cabot's forward foreign exchange contracts are denominated primarily in the Australian dollar, Brazilian real, British pound sterling, Canadian dollar, Euro, Japanese yen, and Malaysian ringgit. The increase in the notional balance of the forward foreign currency contracts relates to the forecast receipt of cash and notes receivable in Japan from the sale of the Company's Supermetals Business.

[2] Cabot's forward foreign exchange contracts designated as cash flow hedges are denominated in Japanese yen.