Accumulated Other Comprehensive Loss - Changes in Each Component of AOCI, Net of Tax (Detail) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Mar. 31, 2015
Accumulated Other Comprehensive Income Loss [Line Items]
Balance attributable to Cabot Corporation	$ (344)	$ (299)
Other comprehensive income (loss) before reclassifications	65	(47)
Amounts reclassified from accumulated other comprehensive (loss) income	1	(1)	$ 1	$ 22
Net other comprehensive items	(278)	(347)
Less: Noncontrolling interest	1	(3)
Balance attributable to Cabot Corporation	(279)	(344)
Currency Translation Adjustment [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance attributable to Cabot Corporation	(283)	(239)
Other comprehensive income (loss) before reclassifications	65	(47)
Net other comprehensive items	(218)	(286)
Less: Noncontrolling interest	1	(3)
Balance attributable to Cabot Corporation	(219)	(283)
Unrealized Gains on Investments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance attributable to Cabot Corporation	2	2
Net other comprehensive items	2	2
Balance attributable to Cabot Corporation	2	2
Pension and Other Postretirement Benefit Liability Adjustments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance attributable to Cabot Corporation	(63)	(62)
Amounts reclassified from accumulated other comprehensive (loss) income	1	(1)
Net other comprehensive items	(62)	(63)
Balance attributable to Cabot Corporation	$ (62)	$ (63)