Unaudited Quarterly Financial Information - Schedule of Quarterly Financial Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Quarterly Financial Information Disclosure [Abstract]
Net sales and other operating revenues	$ 911	$ 940	$ 898	$ 898	$ 896	$ 901	$ 840	$ 819	$ 3,647	$ 3,456
Gross profit	182	184	176	179	167	176	143	147	721	633	644
Selling and administrative expenses	81	76	92	77	75	72	77	73	326	297	281
Research and technical expenses	14	15	16	15	18	17	16	17	60	68	72
Income from operations	87	93	68	87	74	87	50	57	335	268	291
Net interest expense and other charges	(16)	(13)	(20)	22	(18)	(13)	(13)	(14)	(27)	(58)
Income from continuing operations before income taxes and equity in (loss) earnings of affiliated companies	71	80	48	109	56	74	37	43	308	210	246
Provision for income taxes	(41)	(20)	(7)	(24)	(8)	(16)	(16)	(20)	(92)	(60)	(55)
Equity in earnings of affiliated companies	2	(2)	(2)	2	2	3	3	3		11	11
(Loss) income from discontinued operations, net of tax	4	(1)		(1)	1	1	(1)	(2)	2	(1)	204
Net income	36	57	39	86	51	62	23	24	218	160	406
Net income (loss) attributable to noncontrolling interests, net of tax	5	5	3	6	4	3	(4)	4	19	7	18
Net income attributable to Cabot Corporation	$ 31	$ 52	$ 36	$ 80	$ 47	$ 59	$ 27	$ 20	$ 199	$ 153	$ 388
Income per share-basic:
Income from continuing operations	$ 0.43	$ 0.80	$ 0.56	$ 1.25	$ 0.73	$ 0.88	$ 0.43	$ 0.35	$ 3.04	$ 2.39	$ 2.88
(Loss) income from discontinued operations	$ 0.05	$ (0.01)	$ (0.01)	$ (0.01)		$ 0.04	$ (0.01)	$ (0.04)	$ 0.02	$ (0.01)	$ 3.19
Net income attributable to Cabot Corporation	$ 0.48	$ 0.79	$ 0.55	$ 1.24	$ 0.73	$ 0.92	$ 0.42	$ 0.31	$ 3.06	$ 2.38	$ 6.07
Income per share-diluted:
Income from continuing operations	$ 0.43	$ 0.79	$ 0.55	$ 1.24	$ 0.72	$ 0.87	$ 0.43	$ 0.35	$ 3.01	$ 2.37	$ 2.84
(Loss) income from discontinued operations	$ 0.05	$ (0.01)	$ (0.01)	$ (0.01)	$ 0.01	$ 0.03	$ (0.01)	$ (0.04)	$ 0.02	$ (0.01)	$ 3.15
Net income attributable to Cabot Corporation	$ 0.48	$ 0.78	$ 0.54	$ 1.23	$ 0.73	$ 0.90	$ 0.42	$ 0.31	$ 3.03	$ 2.36	$ 5.99