Financial Information By Segment (Schedule Of New Business Segment) (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
Segment Reporting Information [Line Items]
Net sales and other operating revenues	$ 844	[1]	$ 739	[1]	$ 1,606	[1]	$ 1,433	[1]
New Business Segment [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	30		32		55		56
New Business Segment [Member] | Inkjet Colorants Business [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	15		16		30		30
New Business Segment [Member] | Aerogel Business [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	5		8		9		11
New Business Segment [Member] | Cabot Superior MicroPowders Business [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	3		3		5		6
New Business Segment [Member] | Cabot Elastomer Composites Business [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	$ 7		$ 5		$ 11		$ 9

[1] Unallocated and Other reflects royalties paid by equity affiliates, external shipping and handling fees, and other operating revenues, which includes the impact of the corporate adjustment for unearned revenue.