Consolidated Statements of Operations - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Net sales and other operating revenues	$ 2,717	$ 2,411	$ 2,871
Cost of sales	2,065	1,833	2,286
Gross profit	652	578	585
Selling and administrative expenses	260	275	282
Research and technical expenses	56	53	58
Long-lived assets impairment charge (Note E)		23	210
Goodwill impairment charge (Note E)			352
Income (loss) from operations	336	250	(317)
Interest and dividend income	9	5	4
Interest expense	(53)	(54)	(53)
Other income (expense)	(4)	(7)	(11)
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies	288	194	(377)
(Provision) benefit for income taxes	(29)	(34)	45
Equity in earnings of affiliated companies, net of tax	7	3	4
Income (loss) from continuing operations	266	163	(328)
Income (loss) from discontinued operations, net of tax of $—, $1 and $—		1	2
Net income (loss)	266	164	(326)
Net income (loss) attributable to noncontrolling interests, net of tax of $6, $4 and $5	25	15	8
Net income (loss) attributable to Cabot Corporation	$ 241	$ 149	$ (334)
Weighted-average common shares outstanding:
Basic	62.3	62.4	63.4
Diluted	62.7	62.9	63.4
Basic:
Income (loss) from continuing operations attributable to Cabot Corporation	$ 3.83	$ 2.36	$ (5.29)
Income (loss) from discontinued operations		0.02	0.02
Net income (loss) attributable to Cabot Corporation	3.83	2.38	(5.27)
Diluted:
Income (loss) from continuing operations attributable to Cabot Corporation	3.80	2.34	(5.29)
Income (loss) from discontinued operations		0.02	0.02
Net income (loss) attributable to Cabot Corporation	3.80	2.36	(5.27)
Dividends per common share	$ 1.23	$ 1.04	$ 0.88
Purification Solutions [Member]
Long-lived assets impairment charge (Note E)			$ 210
Goodwill impairment charge (Note E)			$ 352