Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Statement [Abstract]
Net sales and other operating revenues	$ 694	$ 940	$ 2,200	$ 2,736
Cost of sales	544	756	1,754	2,197
Gross profit	150	184	446	539
Selling and administrative expenses	67	76	216	245
Research and technical expenses	15	15	44	46
Income from operations	68	93	186	248
Interest and dividend income	1	1	3	3
Interest expense	(13)	(14)	(40)	(41)
Other (expense) income	(3)		(6)	27
Long-lived assets impairment charge (Note F)	(209)		(209)	(4)
Goodwill impairment charge (Note F)	(353)		(353)
(Loss) income from continuing operations	(509)	80	(419)	237
Benefit (provision) for income taxes	64	(20)	47	(51)
Equity in earnings (loss) of affiliated companies, net of tax	1	(2)	4	(2)
(Loss) income from continuing operations	(444)	58	(368)	184
Income (loss) from discontinued operations, net of tax	1	(1)	1	(2)
Net (loss) income	(443)	57	(367)	182
Net income attributable to noncontrolling interests, net of tax	2	5	7	14
Net (loss) income attributable to Cabot Corporation	$ (445)	$ 52	$ (374)	$ 168
Weighted-average common shares outstanding, in millions:
Basic	63.3	64.5	63.7	64.3
Diluted	63.3	65.2	63.7	65.0
Basic:
(Loss) income from continuing operations attributable to Cabot Corporation	$ (7.05)	$ 0.80	$ (5.89)	$ 2.61
Income (loss) from discontinued operations	0.01	(0.01)	0.01	(0.03)
Net (loss) income attributable to Cabot Corporation	(7.04)	0.79	(5.88)	2.58
Diluted:
(Loss) income from continuing operations attributable to Cabot Corporation	(7.05)	0.79	(5.89)	2.58
Income (loss) from discontinued operations	0.01	(0.01)	0.01	(0.03)
Net (loss) income attributable to Cabot Corporation	(7.04)	0.78	(5.88)	2.55
Dividends per common share	$ 0.22	$ 0.22	$ 0.66	$ 0.62