Goodwill and Intangible Assets - Schedule of Goodwill Balances (Detail) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2018	Sep. 30, 2018		Sep. 30, 2017
Goodwill And Intangible Assets [Line Items]
Beginning balance			$ 154
Goodwill acquired	[1]		33
Impairment charge	[2]		(92)
Foreign currency impact			(2)
Ending balance			93
Accumulated impairment losses			(444)		$ (352)
Reinforcement Materials [Member]
Goodwill And Intangible Assets [Line Items]
Beginning balance			53
Foreign currency impact			(1)
Ending balance			52
Performance Chemicals [Member]
Goodwill And Intangible Assets [Line Items]
Beginning balance			9
Goodwill acquired	[1]		33
Foreign currency impact			(1)
Ending balance			41
Purification Solutions [Member]
Goodwill And Intangible Assets [Line Items]
Beginning balance			92
Impairment charge		$ (92)	(92)	[2]
Accumulated impairment losses			$ (444)		$ (352)

[1] Consists of goodwill acquired in the acquisition of Tech Blend as described in Note C.

[2] Refer to Note F for details on the Purification Solutions goodwill impairment test and the resulting impairment charge recorded in the second fiscal quarter of 2018. Based on the Company’s most recent annual goodwill impairment test performed as of May 31, 2018, the fair values of the Reinforcement Materials, Fumed Metal Oxides, and Specialty Compounds reporting units were substantially in excess of their carrying values.