Consolidated Statements of Operations (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Income Statement [Abstract]
Net sales and other operating revenues	$ 3,647	$ 3,456	$ 3,291
Cost of sales	2,926	2,823	2,647
Gross profit	721	633	644
Selling and administrative expenses	326	297	281
Research and technical expenses	60	68	72
Income from operations	335	268	291
Interest and dividend income	3	5	4
Interest expense	(55)	(62)	(46)
Other income (expense)	25	(1)	(3)
Income from continuing operations before income taxes and equity in (loss) earnings of affiliated companies	308	210	246
Provision for income taxes	(92)	(60)	(55)
Equity in earnings of affiliated companies, net of tax		11	11
Income from continuing operations	216	161	202
Income (loss) from discontinued operations, net of tax of $2, $(6) and $116	2	(1)	204
Net income	218	160	406
Net income attributable to noncontrolling interests, net of tax of $5, $5 and $7	19	7	18
Net income attributable to Cabot Corporation	$ 199	$ 153	$ 388
Weighted-average common shares outstanding, in millions:
Basic	64.4	63.8	63.4
Diluted	65.1	64.5	64.2
Basic:
Income from continuing operations attributable to Cabot Corporation	$ 3.04	$ 2.39	$ 2.88
Income (loss) from discontinued operations	$ 0.02	$ (0.01)	$ 3.19
Net income attributable to Cabot Corporation	$ 3.06	$ 2.38	$ 6.07
Diluted:
Income from continuing operations attributable to Cabot Corporation	$ 3.01	$ 2.37	$ 2.84
Income (loss) from discontinued operations	$ 0.02	$ (0.01)	$ 3.15
Net income attributable to Cabot Corporation	$ 3.03	$ 2.36	$ 5.99
Dividends per common share	$ 0.84	$ 0.80	$ 0.76