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Goodwill and Intangible Assets - Schedule of Goodwill Balances (Detail) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Goodwill And Intangible Assets [Line Items]    
Beginning balance $ 133 $ 134
Foreign currency impact 1 (1)
Ending balance 134 133
Reinforcement Materials [Member]    
Goodwill And Intangible Assets [Line Items]    
Beginning balance 48 51
Foreign currency impact 2 (3)
Ending balance 50 48
Performance Chemicals [Member]    
Goodwill And Intangible Assets [Line Items]    
Beginning balance 85 83
Foreign currency impact (1) 2
Ending balance $ 84 $ 85