Contingencies, Commitments, and Other Liabilities (Details) € in Millions, $ in Millions					3 Months Ended			12 Months Ended
	Jul. 23, 2024 USD ($)	Jul. 23, 2024 EUR (€)	Mar. 31, 2023 USD ($)	Mar. 31, 2023 EUR (€)	Mar. 31, 2026 USD ($) location	Dec. 31, 2025 USD ($)	Mar. 31, 2025 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2023 EUR (€)	Apr. 16, 2026 USD ($)	Apr. 16, 2026 EUR (€)	Jul. 23, 2024 EUR (€)
Loss Contingencies [Line Items]
Number of cleanup locations | location					30
Remediation reserve balance					$ 16	$ 16
Remediation reserve balance, classified as a current liability						7	$ 7
Payments related to remediation expenses applied against the reserve (less than)					1		3
Income tax reserve								$ 21	€ 19
Guarantees of third party related to project financing					$ 3
Combined fair value of guarantees						5	$ 5
Indemnification of liability (as a percent)					50.00%
Coverage limit						$ 80
Total amount committed under outstanding surety bonds					$ 43
Surety bonds, amount outstanding					$ 19
Termination notice period (in days)					30 days
Payment timing (in days)					120 days
Supplier Finance Program, Obligation, Current, Statement of Financial Position [Extensible Enumeration]					Accounts payable, trade (Q)	Accounts payable, trade (Q)
Supplier invoices subject to future payment					$ 307	$ 266
Letter of Credit
Loss Contingencies [Line Items]
Maximum borrowing capacity					75
Other Noncurrent Liabilities and Deferred Credits | Separation Agreement
Loss Contingencies [Line Items]
Guarantees of third party related to project financing					$ 1,141	$ 1,141
France | Ministry of the Economy, Finance and Industry, France | Tax Years 2010 Through 2012
Loss Contingencies [Line Items]
Income tax assessment, estimate of possible loss			$ 17	€ 16
Income tax assessment, amount of penalties cancelled	$ 7											€ 7
Recurring Costs of Managing Hazardous Substances and Environmental Programs
Loss Contingencies [Line Items]
Percentage of cost of goods sold (less than as a percent)					1.00%
Income Tax and Interest | France | Ministry of the Economy, Finance and Industry, France | Tax Years 2010 Through 2012
Loss Contingencies [Line Items]
Income tax assessment, estimate of possible loss	$ 10	€ 9	10	9
Income Tax Penalties | France | Ministry of the Economy, Finance and Industry, France | Tax Years 2010 Through 2012
Loss Contingencies [Line Items]
Income tax assessment, estimate of possible loss			7	7
Income Tax Penalties | France | Ministry of the Economy, Finance and Industry, France | Tax Years 2010 Through 2012 | Subsequent Event
Loss Contingencies [Line Items]
Income tax assessment, amount of penalties cancelled										$ 4	€ 3
Additional Interest on Income Tax | France | Ministry of the Economy, Finance and Industry, France | Tax Years 2010 Through 2012
Loss Contingencies [Line Items]
Income tax assessment, estimate of possible loss			$ 2	€ 2
Alcoa Corporation Workers Compensation Claims
Loss Contingencies [Line Items]
Letters of credit, total amount outstanding					$ 43
Arconic Corporation Environmental Obligations
Loss Contingencies [Line Items]
Letters of credit, total amount outstanding					$ 9