Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Income Statement [Abstract]
Net sales and other operating revenues	$ 568	$ 694	$ 1,171	$ 1,506
Cost of sales	418	555	922	1,210
Gross profit	150	139	249	296
Selling and administrative expenses	62	71	133	149
Research and technical expenses	11	14	27	29
Income from operations	77	54	89	118
Interest and dividend income	2	1	3	2
Interest expense	(14)	(14)	(27)	(27)
Other expense	(3)	(2)	(11)	(3)
Income from continuing operations	62	39	54	90
Provision for income taxes	(11)	(14)	(6)	(17)
Equity in earnings of affiliated companies, net of tax	1	2	1	3
Net income	52	27	49	76
Net income attributable to noncontrolling interests, net of tax	4	1	8	5
Net income attributable to Cabot Corporation	$ 48	$ 26	$ 41	$ 71
Weighted-average common shares outstanding:
Basic	62.4	63.6	62.4	63.9
Diluted	62.8	64.1	62.9	64.4
Basic:
Net income attributable to Cabot Corporation	$ 0.76	$ 0.41	$ 0.65	$ 1.11
Diluted:
Net income attributable to Cabot Corporation	0.76	0.41	0.65	1.10
Dividends per common share	$ 0.22	$ 0.22	$ 0.44	$ 0.44