Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Net sales and other operating revenues	$ 611	$ 603
Cost of sales	454	504
Gross profit	157	99
Selling and administrative expenses	63	71
Research and technical expenses	12	16
Income (loss) from operations	82	12
Interest and dividend income	2	1
Interest expense	(13)	(13)
Other income (expense)	2	(8)
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies	73	(8)
(Provision) benefit for income taxes	(17)	5
Equity in earnings of affiliated companies, net of tax	2
Net income (loss)	58	(3)
Net income (loss) attributable to noncontrolling interests, net of tax	4	4
Net income (loss) attributable to Cabot Corporation	$ 54	$ (7)
Weighted-average common shares outstanding:
Basic	62.2	62.5
Diluted	62.8	62.5
Basic:
Net income (loss) attributable to Cabot Corporation	$ 0.85	$ (0.11)
Diluted:
Net income (loss) attributable to Cabot Corporation	0.85	(0.11)
Dividends per common share	$ 0.30	$ 0.22