Contingencies, Commitments, and Other Liabilities (Details) € in Millions, $ in Millions							3 Months Ended		6 Months Ended	12 Months Ended
	Apr. 16, 2026 USD ($)	Apr. 16, 2026 EUR (€)	Jul. 23, 2024 USD ($)	Jul. 23, 2024 EUR (€)	Mar. 31, 2023 USD ($)	Mar. 31, 2023 EUR (€)	Jun. 30, 2026 USD ($)	Dec. 31, 2025 USD ($)	Jun. 30, 2026 USD ($) location	Dec. 31, 2023 USD ($)	Dec. 31, 2023 EUR (€)
Loss Contingencies [Line Items]
Number of cleanup locations | location									30
Remediation reserve balance							$ 16	$ 16	$ 16
Remediation reserve balance, classified as a current liability							7	$ 7	7
Payments related to remediation expenses applied against the reserve (less than)							1		1
Income tax reserve										$ 21	€ 19
Total amount committed under outstanding surety bonds							43		43
Surety bonds, amount outstanding							$ 19		$ 19
Termination notice period (in days)									30 days
Payment timing (in days)							120 days		120 days
Supplier Finance Program, Obligation, Current, Statement of Financial Position [Extensible Enumeration]							Accounts payable, trade (Q)	Accounts payable, trade (Q)	Accounts payable, trade (Q)
Supplier invoices subject to future payment							$ 377	$ 266	$ 377
Combined fair value of guarantees							5	5	$ 5
Indemnification of liability (as a percent)									50.00%
Coverage limit								80
Guarantees of third party related to project financing							4		$ 4
Letter of Credit
Loss Contingencies [Line Items]
Maximum borrowing capacity							76		76
Location, Statement of Financial Position, Balance [Axis]: us-gaap:DeferredCreditsAndOtherLiabilitiesNoncurrent | Separation Agreement
Loss Contingencies [Line Items]
Guarantees of third party related to project financing							1,141	$ 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			$ 7	€ 7	$ 17	€ 16
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	$ 4	€ 3			7	7
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		$ 43
Arconic Corporation Environmental Obligations
Loss Contingencies [Line Items]
Letters of credit, total amount outstanding							$ 8		$ 8