Consolidated Statements of Operations - USD ($) shares in Millions, $ in Millions		12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Net sales and other operating revenues		$ 3,242	$ 2,717	$ 2,411
Cost of sales		2,461	2,054	1,836
Gross profit		781	663	575
Selling and administrative expenses		305	260	275
Research and technical expenses		66	56	53
Long-lived assets impairment charge (Note F)		162		23
Goodwill impairment charge (Note F)	[1]	92
Income (loss) from operations		156	347	247
Interest and dividend income		10	9	5
Interest expense		(54)	(53)	(54)
Other income (expense)		5	(4)	(7)
Income (loss) from continuing operations before income taxes and equity in earnings of affiliated companies		117	299	191
(Provision) benefit for income taxes		(193)	(33)	(33)
Equity in earnings of affiliated companies, net of tax		2	7	3
Income (loss) from continuing operations		(74)	273	161
Income (loss) from discontinued operations, net of tax of $—, $— and $1				1
Net income (loss)		(74)	273	162
Net income (loss) attributable to noncontrolling interests, net of tax of $10, $6 and $4		39	25	15
Net income (loss) attributable to Cabot Corporation		$ (113)	$ 248	$ 147
Weighted-average common shares outstanding:
Basic		61.7	62.3	62.4
Diluted		61.7	62.7	62.9
Basic:
Income (loss) from continuing operations attributable to Cabot Corporation		$ (1.85)	$ 3.94	$ 2.32
Income (loss) from discontinued operations				0.02
Net income (loss) attributable to Cabot Corporation		(1.85)	3.94	2.34
Diluted:
Income (loss) from continuing operations attributable to Cabot Corporation		(1.85)	3.91	2.30
Income (loss) from discontinued operations				0.02
Net income (loss) attributable to Cabot Corporation		(1.85)	3.91	2.32
Dividends per common share		$ 1.29	$ 1.23	$ 1.04
Purification Solutions [Member]
Long-lived assets impairment charge (Note F)		$ 162
Goodwill impairment charge (Note F)	[1]	$ 92

[1] Refer to Note F for details on the Purification Solutions goodwill impairment test and the resulting impairment charge recorded in the second fiscal quarter of 2018. Based on the Company’s most recent annual goodwill impairment test performed as of May 31, 2018, the fair values of the Reinforcement Materials, Fumed Metal Oxides, and Specialty Compounds reporting units were substantially in excess of their carrying values.