Consolidated Statements of Operations (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Net sales and other operating revenues	$ 812	$ 898
Cost of sales	655	719
Gross profit	157	179
Selling and administrative expenses	78	77
Research and technical expenses	15	15
Income from operations	64	87
Interest and dividend income	1	1
Interest expense	(13)	(14)
Other (expense) income	(1)	35
Income from continuing operations before income taxes and equity in earnings of affiliated companies	51	109
Provision for income taxes	(3)	(24)
Equity in earnings of affiliated companies, net of tax	1	2
Income from continuing operations	49	87
Loss from discontinued operations, net of tax		(1)
Net income	49	86
Net income attributable to noncontrolling interests, net of tax	4	6
Net income attributable to Cabot Corporation	$ 45	$ 80
Weighted-average common shares outstanding, in millions:
Basic	64.1	64.2
Diluted	64.6	64.8
Basic:
Income from continuing operations attributable to Cabot Corporation	$ 0.70	$ 1.25
Loss from discontinued operations		$ (0.01)
Net income attributable to Cabot Corporation	$ 0.70	$ 1.24
Diluted:
Income from continuing operations attributable to Cabot Corporation	$ 0.69	$ 1.24
Loss from discontinued operations		$ (0.01)
Net income attributable to Cabot Corporation	$ 0.69	$ 1.23
Dividends per common share	$ 0.22	$ 0.20