Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ (221)	$ 186	$ (74)
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax	42	(69)	(64)
Unrealized holding gains (losses) arising during the period, net of tax			(1)
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense, net of tax	(5)	(4)	(3)
(Gains) losses excluded from effectiveness testing and amortized to interest expense, net of tax	2	1	1
Pension and other postretirement benefit liability adjustments, net of tax	9	(5)	5
Specialty Fluids divestiture		(3)
Other comprehensive income (loss)	48	(80)	(62)
Comprehensive income (loss)	(173)	106	(136)
Net income (loss) attributable to noncontrolling interests, net of tax	17	29	39
Foreign currency translation adjustment attributable to noncontrolling interests, net of tax	5	(6)	(4)
Comprehensive income (loss) attributable to noncontrolling interests	22	23	35
Comprehensive income (loss) attributable to Cabot Corporation	$ (195)	$ 83	$ (171)