Goodwill and Intangible Assets - Schedule of Changes in Balances (Detail) $ in Millions	9 Months Ended
Jun. 30, 2026 USD ($)
Goodwill And Intangible Assets [Line Items]
Beginning balance	$ 134
Foreign currency impact	3
Ending balance	137
Reinforcement Materials [Member]
Goodwill And Intangible Assets [Line Items]
Beginning balance	50
Foreign currency impact	2
Ending balance	52
Performance Chemicals [Member]
Goodwill And Intangible Assets [Line Items]
Beginning balance	84
Foreign currency impact	1
Ending balance	$ 85

Amount, after accumulated impairment loss, of asset representing future economic benefit arising from other asset acquired in business combination or from joint venture formation or both, that is not individually identified and separately recognized.

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+ Details