Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Net sales and other operating revenues	$ 705	$ 621	$ 1,994	$ 1,792
Cost of sales	546	461	1,509	1,383
Gross profit	159	160	485	409
Selling and administrative expenses	63	64	191	197
Research and technical expenses	14	13	40	40
Income (loss) from operations	82	83	254	172
Interest and dividend income	3	1	7	4
Interest expense	(13)	(13)	(39)	(40)
Other income (expense)	(6)	3	(5)	(8)
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies	66	74	217	128
(Provision) benefit for income taxes	(16)	(15)	(32)	(21)
Equity in earnings of affiliated companies, net of tax	3	1	6	2
Net income (loss)	53	60	191	109
Net income (loss) attributable to noncontrolling interests, net of tax	8	4	18	12
Net income (loss) attributable to Cabot Corporation	$ 45	$ 56	$ 173	$ 97
Weighted-average common shares outstanding:
Basic	62.4	62.4	62.3	62.4
Diluted	62.7	62.9	62.8	62.9
Earnings per common share:
Basic	$ 0.71	$ 0.90	$ 2.75	$ 1.55
Diluted	0.71	0.88	2.74	1.53
Dividends per common share	$ 0.315	$ 0.30	$ 0.915	$ 0.74