Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Net sales and other operating revenues	$ 727	$ 821
Cost of sales	586	655
Gross profit	141	166
Selling and administrative expenses	64	73
Research and technical expenses	14	16
Income (loss) from operations	63	77
Interest and dividend income	3	2
Interest expense	(14)	(15)
Other income (expense)	(2)	6
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies	50	70
(Provision) benefit for income taxes	(4)	7
Net income (loss)	46	77
Net income (loss) attributable to noncontrolling interests, net of tax	5	8
Net income (loss) attributable to Cabot Corporation	$ 41	$ 69
Weighted-average common shares outstanding:
Basic	56.9	59.9
Diluted	57.0	60.1
Earnings per common share:
Basic	$ 0.71	$ 1.14
Diluted	$ 0.70	$ 1.14