Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 286	$ (221)	$ 186
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax	52	42	(69)
Derivatives: net investment hedges
(Gains) losses reclassified to interest expense, net of tax	(5)	(5)	(4)
(Gains) losses excluded from effectiveness testing and amortized to interest expense, net of tax	2	2	1
Pension and other postretirement benefit liability adjustments, net of tax	20	9	(5)
Specialty Fluids divestiture			(3)
Other comprehensive income (loss), net of tax of $8, $1 and $2	69	48	(80)
Comprehensive income (loss)	355	(173)	106
Net income (loss) attributable to noncontrolling interests, net of tax	36	17	29
Foreign currency translation adjustment attributable to noncontrolling interests, net of tax	7	5	(6)
Comprehensive income (loss) attributable to noncontrolling interests	43	22	23
Comprehensive income (loss) attributable to Cabot Corporation	$ 312	$ (195)	$ 83