Goodwill and Intangible Assets - Schedule of Goodwill Balances (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Goodwill And Intangible Assets [Line Items]
Beginning balance	$ 134	$ 129
Foreign currency impact	(1)	5
Ending balance	133	134
Reinforcement Materials [Member]
Goodwill And Intangible Assets [Line Items]
Beginning balance	51	46
Foreign currency impact	(3)	5
Ending balance	48	51
Performance Chemicals [Member]
Goodwill And Intangible Assets [Line Items]
Beginning balance	83	83
Foreign currency impact	2
Ending balance	$ 85	$ 83