Financial instruments - Additional information (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2026	Dec. 31, 2025
Disclosure of detailed information about financial instruments [line items]
Borrowings	$ 21,414	$ 21,931
Financial and general covenants	$ 3,451	3,795
Level 3
Disclosure of detailed information about financial instruments [line items]
Percentage change in forecast electricity prices	10.00%
Increase in carrying value of renewable power purchase agreements due to forecast electricity prices	$ 540	520
Decrease in carrying value of renewable power purchase agreements due to forecast electricity prices	540	521
Level 3 \| Discounted cash flows \| Forward Electricity Price And Energy Volume \| Renewable Power Purchase Agreements
Disclosure of detailed information about financial instruments [line items]
Financial liabilities, at fair value	248	29
Level 3 \| Option pricing model \| LME Forward Aluminum Price And Midwest Premium And Billet Premium, Measurement Input \| Derivatives Embedded In Electricity Contracts
Disclosure of detailed information about financial instruments [line items]
Financial liabilities, at fair value	$ (284)	$ (320)

The amount of outstanding funds that the entity is obligated to repay.

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The fair value of financial liabilities. [Refer: At fair value [member]; Financial liabilities]

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Debt Instruments Restrictive Covenants

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