Financial Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Derivative [Line Items]
Maximum remaining maturity of foreign currency derivatives, days	30		30
Number of open derivatives designated as net investment hedges			0
Foreign currency exchange rate, remeasurement income (loss)	$ 4	$ 11	$ (5)	$ 6
Bond held by one of Cabot's European subsidiaries	175	175	175	175
Forward Foreign Currency Contracts [Member]
Derivative [Line Items]
Gain (loss) on foreign currency derivative instruments not designated as hedging instruments	10	(2)	9	1
Currency Swaps [Member]
Derivative [Line Items]
Gain (loss) on foreign currency derivative instruments not designated as hedging instruments	$ (3)	$ (7)	$ 5	$ (3)