Contingencies and Commitments - Commitments (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2024
Loss Contingencies [Line Items]
Purchase obligations due in 2025	$ 237			$ 237
Purchase obligations due in 2026	51			51
Purchase obligations due in 2027	42			42
Purchase obligations due in 2028	40			40
Purchase obligations due in 2029	39			39
Purchase obligations due thereafter	0			$ 0
Indemnification of liability (as a percent)				50.00%
Guarantees of third party related to project financing	6			$ 6
Combined fair value of guarantees	6	$ 6		6
Insurance annual coverage limit	80	80	$ 80
Total amount committed under outstanding surety bonds	44			44
Amount of outstanding surety bonds relating to these liabilities	21			21
Alcoa Corporation Workers Compensation Claims
Loss Contingencies [Line Items]
Letters of credit, total amount committed	48			48
Arconic Corporation Environmental Obligations
Loss Contingencies [Line Items]
Letters of credit, total amount committed	17			17
Letter of Credit
Loss Contingencies [Line Items]
Maximum borrowing capacity	90			90
Other Noncurrent Liabilities and Deferred Credits | Separation Agreement
Loss Contingencies [Line Items]
Guarantees of third party related to project financing	$ 1,121	$ 1,131		$ 1,121