Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Net sales and other operating revenues	$ 821	$ 720
Cost of sales	655	544
Gross profit	166	176
Selling and administrative expenses	73	70
Research and technical expenses	16	15
Income (loss) from operations	77	91
Interest and dividend income	2	3
Interest expense	(15)	(13)
Other income (expense)	6	11
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies	70	92
(Provision) benefit for income taxes	7	(205)
Equity in earnings of affiliated companies, net of tax		1
Net income (loss)	77	(112)
Net income (loss) attributable to noncontrolling interests, net of tax	8	10
Net income (loss) attributable to Cabot Corporation	$ 69	$ (122)
Weighted-average common shares outstanding:
Basic	59.9	61.9
Diluted	60.1	61.9
Earnings per common share:
Basic	$ 1.14	$ (1.98)
Diluted	1.14	(1.98)
Dividends per common share	$ 0.33	$ 0.315