Consolidated Statements Of Operations (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Consolidated Statements Of Operations [Abstract]
Net sales and other operating revenues	$ 762	$ 694
Cost of sales	619	563
Gross profit	143	131
Selling and administrative expenses	65	63
Research and technical expenses	17	15
Income from operations	61	53
Interest and dividend income	1	1
Interest expense	(10)	(10)
Other income	3	2
Income from continuing operations before income taxes and equity in net earnings of affiliated companies	55	46
(Provision) benefit for income taxes	(16)	15
Equity in net earnings of affiliated companies	1	3
Income from continuing operations	40	64
Income from discontinued operations, net of tax	11	16
Net income	51	80
Net income attributable to noncontrolling interests, net of tax	5	5
Net income attributable to Cabot Corporation	$ 46	$ 75
Weighted-average common shares outstanding, in millions:
Basic	63.5	64.4
Diluted	64.2	65.2
Basic:
Income from continuing operations attributable to Cabot Corporation	$ 0.55	$ 0.89
Income from discontinued operations	$ 0.17	$ 0.25
Net income attributable to Cabot Corporation	$ 0.72	$ 1.14
Diluted:
Income from continuing operations attributable to Cabot Corporation	$ 0.55	$ 0.88
Income from discontinued operations	$ 0.16	$ 0.25
Net income attributable to Cabot Corporation	$ 0.71	$ 1.13
Dividends per common share	$ 0.18	$ 0.18