Purification Solutions Goodwill and Long-Lived Assets Impairment Charges - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2018		Sep. 30, 2016
Goodwill And Intangible Assets [Line Items]
Goodwill impairment charge	[1]			$ 92
Lower of cost or market reserve inventory		$ 13		13
Long-lived assets impairment charge				162		$ 23
Assets impairment charges income tax benefit			$ 30
Reducing of impairment charges and restructuring costs		$ 1
Purification Solutions [Member]
Goodwill And Intangible Assets [Line Items]
Goodwill impairment charge			92	92	[1]
Lower of cost or market reserve inventory			13
Long-lived assets impairment charge			162	$ 162
Impairment charges of definite lived Intangible assets			64
Purification Solutions [Member] | Property, Plant and Equipment [Member]
Goodwill And Intangible Assets [Line Items]
Long-lived assets impairment charge			$ 98

[1] 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.