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Fair Value Measurements - Level 3 Reconciliation (Details) $ in Millions	12 Months Ended
	Dec. 31, 2022 USD ($) MT t	Dec. 31, 2021 USD ($) MT
LME forward financial sales contracts
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance as of January 1, 2022	$ (5.1)
Purchases, sales, settlements
Balance as of December 31, 2022	$ (2.8)	$ (5.1)
LME forward financial sales contracts | Level 3
Purchases, sales, settlements
Derivative asset, percentage	0.0858
LME forward financial sales contracts | Level 3 | Discounted Cash Flow
Purchases, sales, settlements
Derivative asset, percentage		0.0858
FX Swap
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance as of January 1, 2022	$ (0.2)	$ 0.1
Total realized/unrealized gains (losses)
Included in net income (loss)	0.0	(0.6)
Purchases, sales, settlements
Purchases	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Transfers into Level 3	0.0	(0.4)
Transfers out of Level 3	0.2	0.7
Balance as of December 31, 2022	0.0	(0.2)
Change in unrealized gains (losses)	$ 0.0	$ (0.6)
FX Swap | Level 3
Purchases, sales, settlements
Derivative asset, percentage	0.0858
FX Swap | Level 3 | Discounted Cash Flow
Purchases, sales, settlements
Derivative asset, percentage		0.0858
Nordpool Power Price Swap
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance as of January 1, 2022	$ 0.2	$ 0.0
Total realized/unrealized gains (losses)
Included in net income (loss)		0.0
Purchases, sales, settlements
Purchases		0.0
Sales		0.0
Settlements		0.0
Transfers into Level 3		1.7
Transfers out of Level 3		(1.5)
Balance as of December 31, 2022	0.0	0.2
Change in unrealized gains (losses)		0.0
Nordpool Power Price Swap | Level 3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance as of January 1, 2022	0.2
Total realized/unrealized gains (losses)
Included in net income (loss)	0.0
Purchases, sales, settlements
Purchases	0.0
Sales	0.0
Settlements	0.0
Transfers into Level 3	0.0
Transfers out of Level 3	(0.2)
Balance as of December 31, 2022	$ 0.0	$ 0.2
Derivative asset, percentage	0.0858
Change in unrealized gains (losses)	$ 0.0
Nordpool Power Price Swap | Level 3 | Measurement Input, Discount Rate
Purchases, sales, settlements
Derivative asset, percentage		0.0858
Casthouse
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance as of January 1, 2022	0.0
Purchases, sales, settlements
Balance as of December 31, 2022	$ 0.0	$ 0.0
Casthouse | Level 3 | Measurement Input, Discount Rate
Purchases, sales, settlements
Derivative asset, percentage	0.0858	0
Contingent obligation
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance as of January 1, 2022	$ 0.0
Purchases, sales, settlements
Balance as of December 31, 2022	$ 0.0	$ 0.0
Contingent obligation | Level 3 | Minimum
Purchases, sales, settlements
Open position to offset fixed prices | MT	0	14,000
Contingent obligation | Level 3 | Maximum
Purchases, sales, settlements
Open position to offset fixed prices | MT		15,000
LME forward financial sales contracts
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance as of January 1, 2022	$ 0.0
Total realized/unrealized gains (losses)
Included in net income (loss)	1.6
Purchases, sales, settlements
Purchases	0.0
Sales	0.0
Settlements	0.0
Transfers into Level 3	1.7
Transfers out of Level 3	(1.5)
Balance as of December 31, 2022	1.8	$ 0.0
Change in unrealized gains (losses)	1.6
LME Swap
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance as of January 1, 2022	(5.1)	2.9
Total realized/unrealized gains (losses)
Included in net income (loss)	5.3	(28.7)
Purchases, sales, settlements
Purchases	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Transfers into Level 3	(2.5)	(1.0)
Transfers out of Level 3	(2.3)	21.7
Balance as of December 31, 2022	(4.6)	(5.1)
Change in unrealized gains (losses)	$ 5.3	(28.7)
Open position to offset fixed prices | t	111,741
Casthouse Hedges
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance as of January 1, 2022	$ 0.0
Total realized/unrealized gains (losses)
Included in net income (loss)	0.0
Purchases, sales, settlements
Purchases	0.0
Sales	0.0
Settlements	0.0
Transfers into Level 3	0.0
Transfers out of Level 3	0.0
Balance as of December 31, 2022	0.0	$ 0.0
Change in unrealized gains (losses)	$ 0.0

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Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Total Realized/Unrealized Gains (Losses) [Abstract]

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Value of input used to measure derivative asset.

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Nominal mass used to calculate payments on a derivative instrument.

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Amount of gain (loss) recognized in income from derivative asset (liability) after deduction of derivative liability (asset), measured at fair value on recurring basis using unobservable input (level 3).

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Amount of purchases of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of sales of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of transfers of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset) into level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset) out of level 3 of the fair value hierarchy.

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