Accumulated Other Comprehensive Loss - Changes in Each Component of AOCI, Net of Tax (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance attributable to Cabot Corporation	$ (64)
Other comprehensive loss before reclassifications	(103)
Amounts reclassified from accumulated other comprehensive income	21
Net other comprehensive items	(82)	(21)
Less: Noncontrolling interest	(2)
Balance attributable to Cabot Corporation	(144)
Currency Translation Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance attributable to Cabot Corporation	27
Other comprehensive loss before reclassifications	(103)
Net other comprehensive items	(103)
Less: Noncontrolling interest	(2)
Balance attributable to Cabot Corporation	(74)
Unrealized Gains on Investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance attributable to Cabot Corporation			2
Balance attributable to Cabot Corporation	2		2
Pension and Other Postretirement Benefit Liability Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance attributable to Cabot Corporation	(93)
Amounts reclassified from accumulated other comprehensive income	21
Net other comprehensive items	21
Balance attributable to Cabot Corporation	$ (72)