Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Income Statement [Abstract]
Net sales and other operating revenues	$ 678	$ 568	$ 1,289	$ 1,171
Cost of sales	509	418	963	922
Gross profit	169	150	326	249
Selling and administrative expenses	65	62	128	133
Research and technical expenses	14	11	26	27
Income (loss) from operations	90	77	172	89
Interest and dividend income	2	2	4	3
Interest expense	(13)	(14)	(26)	(27)
Other income (expense)	(1)	(3)	1	(11)
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies	78	62	151	54
(Provision) benefit for income taxes	1	(11)	(16)	(6)
Equity in earnings of affiliated companies, net of tax	1	1	3	1
Net income (loss)	80	52	138	49
Net income (loss) attributable to noncontrolling interests, net of tax	6	4	10	8
Net income (loss) attributable to Cabot Corporation	$ 74	$ 48	$ 128	$ 41
Weighted-average common shares outstanding:
Basic	62.4	62.4	62.3	62.4
Diluted	62.8	62.8	62.8	62.9
Earnings per common share:
Basic	$ 1.19	$ 0.76	$ 2.04	$ 0.65
Diluted	1.18	0.76	2.03	0.65
Dividends per common share	$ 0.30	$ 0.22	$ 0.60	$ 0.44