Revenue - Disaggregation of Revenue by Product Category (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Disaggregation of Revenue [Line Items]
Net sales	$ 752.1	$ 628.1	$ 1,401.3	$ 1,262.0
Aluminum
Disaggregation of Revenue [Line Items]
Net sales	701.9	568.6	1,289.8	1,104.4
Alumina
Disaggregation of Revenue [Line Items]
Net sales	$ 50.2	$ 59.5	$ 111.5	$ 157.6

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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