Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Net sales and other operating revenues	$ 720	$ 611
Cost of sales	542	452
Gross profit	178	159
Selling and administrative expenses	69	63
Research and technical expenses	15	12
Income (loss) from operations	94	84
Interest and dividend income	3	2
Interest expense	(13)	(13)
Other income (expense)	8	2
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies	92	75
(Provision) benefit for income taxes	(205)	(18)
Equity in earnings of affiliated companies, net of tax	1	2
Net income (loss)	(112)	59
Net income (loss) attributable to noncontrolling interests, net of tax	10	4
Net income (loss) attributable to Cabot Corporation	$ (122)	$ 55
Weighted-average common shares outstanding:
Basic	61.9	62.2
Diluted	61.9	62.8
Earnings per common share:
Basic	$ (1.98)	$ 0.87
Diluted	(1.98)	0.86
Dividends per common share	$ 0.315	$ 0.30