Unaudited Quarterly Financial Information - Schedule of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Quarterly Financial Information Disclosure [Abstract]
Net sales and other operating revenues	$ 723	$ 705	$ 678	$ 611	$ 619	$ 621	$ 568	$ 603	$ 2,717	$ 2,411	$ 2,871
Gross profit	167	159	169	157	169	160	150	99	652	578	585
Income (loss) from discontinued operations, net of tax					1					1	2
Net income (loss)	75	53	80	58	55	60	52	(3)	266	164	(326)
Net income (loss) attributable to Cabot Corporation	$ 68	$ 45	$ 74	$ 54	$ 52	$ 56	$ 48	$ (7)	$ 241	$ 149	$ (334)
Basic:
Income (loss) from continuing operations					$ 0.81	$ 0.90	$ 0.76	$ (0.11)	$ 3.83	$ 2.36	$ (5.29)
Income (loss) from discontinued operations					0.02					0.02	0.02
Net income (loss) attributable to Cabot Corporation	$ 1.08	$ 0.71	$ 1.19	$ 0.85	0.83	0.90	0.76	(0.11)	3.83	2.38	(5.27)
Diluted:
Income (loss) from continuing operations					0.81	0.88	0.76	(0.11)	3.80	2.34	(5.29)
Income (loss) from discontinued operations					0.02					0.02	0.02
Net income (loss) attributable to Cabot Corporation	$ 1.06	$ 0.71	$ 1.18	$ 0.85	$ 0.83	$ 0.88	$ 0.76	$ (0.11)	$ 3.80	$ 2.36	$ (5.27)