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Fair Value Measurements - Level 3 Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
LME forward financial sales contracts
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance, beginning	$ 0.0	$ 1.8	$ 0.0
Total realized/unrealized gains (losses)
Included in net income	0.0		0.0
Transfers into Level 3	1.6		1.6
Transfers out of Level 3		(1.8)	0.0
Balance, ending	1.6	0.0	1.6
Change in unrealized gains (losses)	0.0	0.0	0.0
LME forward financial sales contracts
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance, beginning	(16.7)	(4.6)	(5.1)
Total realized/unrealized gains (losses)
Included in net income	12.9		3.8
Transfers into Level 3	0.0		(2.5)
Transfers out of Level 3		4.6	0.0
Balance, ending	(3.8)	0.0	(3.8)
Change in unrealized gains (losses)	12.9	$ 0.0	3.8
Casthouse currency hedges
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance, beginning	0.0		0.0
Total realized/unrealized gains (losses)
Included in net income	0.0		0.0
Transfers into Level 3	(0.0)		0.0
Transfers out of Level 3			0.0
Balance, ending	0.0		0.0
Change in unrealized gains (losses)	0.0		0.0
Nord Pool Swaps
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance, beginning			0.2
Total realized/unrealized gains (losses)
Included in net income			0.0
Transfers into Level 3			0.0
Transfers out of Level 3			(0.2)
Balance, ending	0.0		0.0
Change in unrealized gains (losses)			0.0
FX Swaps
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance, beginning			(0.2)
Total realized/unrealized gains (losses)
Included in net income			0.0
Transfers into Level 3			0.0
Transfers out of Level 3			0.2
Balance, ending	$ 0.0		0.0
Change in unrealized gains (losses)			$ 0.0

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- Definition

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

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Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Unrealized Gains (Losses)

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
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- Definition

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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-Topic 820
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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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- Definition

Fair value of financial instrument classified as 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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