Recent Accounting Pronouncements - Additional Information (Detail) - USD ($) $ in Millions	Oct. 01, 2019	Sep. 30, 2020	Sep. 30, 2019
Significant Accounting Policies [Line Items]
Operating lease right-of-use assets	$ 106	$ 98
Operating lease liability	$ 111	$ 104
Operating Lease, Liability, Statement of Financial Position [Extensible List]	cbt:AccountsPayableAndAccruedLiabilitiesAndOtherLiabilitiesMember	cbt:AccountsPayableAndAccruedLiabilitiesAndOtherLiabilitiesMember
Reclassification of deferred rent balances	$ 5
Cumulative adjustment to retained earnings		$ 989	$ 1,337
Reclassification of net gain from Accumulated other comprehensive income (loss) to retained earnings	2
Cumulative Effect, Period of Adoption, Adjustment [Member] | Revision of Prior Period, Adjustment [Member]
Significant Accounting Policies [Line Items]
Cumulative adjustment to retained earnings	$ 0