Contingencies and Commitments - Additional Information (Detail) $ in Millions			6 Months Ended	12 Months Ended
	Oct. 15, 2024 USD ($)	Jul. 18, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2024 USD ($) Project	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Loss Contingencies [Line Items]
Liabilities incurred				$ 25	$ 39	$ 32
Payments against the reserve				49	55	26
Reversals of previously recorded liabilities				12	1	30
Environmental remediation reserve balance, current			$ 66	$ 38	$ 66
Environmental Loss Contingency, Current, Statement of Financial Position [Extensible Enumeration]			Other current liabilities	Other current liabilities	Other current liabilities
Active or future remediation for significant sites			$ 211	$ 154	$ 211
Accrued environmental reserves			268	220	268	284	$ 309
Payment for settlements to former employees	$ 5	$ 5	76
Ongoing Remediation Work [Member]
Loss Contingencies [Line Items]
Liabilities incurred				$ 20
Former East St. Louis Site [Member]
Loss Contingencies [Line Items]
Liabilities incurred						4
Magnesium Smelter Facility in Addy [Member]
Loss Contingencies [Line Items]
Liabilities incurred						14
Point Henry Site [Member]
Loss Contingencies [Line Items]
Liabilities incurred						6
Environmental activiteis
Loss Contingencies [Line Items]
Liabilities incurred						9
Intalco Aluminum Smelter [Member]
Loss Contingencies [Line Items]
Liabilities incurred					14
Massena East Site [Member]
Loss Contingencies [Line Items]
Reversals of previously recorded liabilities						18
Suralco [Member]
Loss Contingencies [Line Items]
Reversals of previously recorded liabilities						5
Closed Site in Brazil [Member]
Loss Contingencies [Line Items]
Reversals of previously recorded liabilities						$ 6
Massena, New York [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Environmental remediation work completion period				8 years
Previously Closed Sites [Member] | Ongoing Remediation Work [Member]
Loss Contingencies [Line Items]
Liabilities incurred				$ 5
Longview, Washington [Member] | Intalco Aluminum Smelter [Member] | Ongoing Remediation Work [Member]
Loss Contingencies [Line Items]
Liabilities incurred					13
Addy, Washington [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Environmental remediation work completion period				3 years
Addy, Washington [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Environmental remediation work completion period				5 years
Ferndale Washington [Member]
Loss Contingencies [Line Items]
Post-closure maintenance and monitoring period				5 years
Other Sites [Member]
Loss Contingencies [Line Items]
Liabilities incurred					12
Number of remediation projects | Project				31
Accrued environmental reserves			$ 57	$ 66	$ 57