Investments - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2013
Alcoa Corporation [Member]
Schedule Of Equity Method Investments [Line Items]
Charges recorded		$ 21,000,000	$ 62,000,000
Alcoa Corporation [Member] | Other Nonoperating Income Expense [Member]
Schedule Of Equity Method Investments [Line Items]
Adjustment recorded	$ 41,000,000
ELYSIS TM Limited Partnership [Member]
Schedule Of Equity Method Investments [Line Items]
Basis in investment, due to share of losses				$ 0	$ 0
Unrecognized losses				$ 54,000,000