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Fair Value Measurements - Level 3 Reconciliation (Details) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2021 USD ($)	Dec. 31, 2021 USD ($) MT t	Dec. 31, 2020 USD ($) MT
LME Swap
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance as of January 1, 2021		$ 2.9	$ 10.6
Total realized/unrealized gains (losses)
Included in net income (loss)		(28.7)	(1.5)
Purchases, sales, settlements
Purchases		0.0	0.0
Sales		0.0	0.0
Settlements		0.0	0.0
Transfers into Level 3		(1.0)	6.5
Transfers out of Level 3		21.7	(12.7)
Balance as of December 31, 2021	$ (5.1)	(5.1)	2.9
Change in unrealized gains (losses)		$ (28.7)	(1.5)
Open position to offset fixed prices | t		123,216
LME Swap | Level 3
Purchases, sales, settlements
Derivative asset, percentage	0.0858	0.0858
FX Swap
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance as of January 1, 2021		$ 0.1	0.0
Total realized/unrealized gains (losses)
Included in net income (loss)	$ (0.6)		0.0
Purchases, sales, settlements
Purchases			0.0
Sales			0.0
Settlements			0.0
Transfers into Level 3		(0.4)	0.1
Transfers out of Level 3		0.7	0.0
Balance as of December 31, 2021	(0.2)	$ (0.2)	0.1
Change in unrealized gains (losses)	$ (0.6)		0.0
FX Swap | Level 3
Purchases, sales, settlements
Derivative asset, percentage	0.0858	0.0858
Hawesville L4 Power Price Swaps
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance as of January 1, 2021		$ 0.0	(2.9)
Total realized/unrealized gains (losses)
Included in net income (loss)			0.1
Purchases, sales, settlements
Purchases			0.0
Sales			0.0
Settlements			2.8
Transfers into Level 3			0.0
Transfers out of Level 3			0.0
Balance as of December 31, 2021			0.0
Change in unrealized gains (losses)			2.9
Nordpool Power Price Swap
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance as of January 1, 2021		0.0
Purchases, sales, settlements
Balance as of December 31, 2021	$ 0.2	0.2	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, 2021		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, 2021	$ 0.2	$ 0.2	0.0
Derivative asset, percentage	0.0858	0.0858
Change in unrealized gains (losses)	$ 0.0
Contingent obligation
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance as of January 1, 2021		$ 0.0
Purchases, sales, settlements
Balance as of December 31, 2021	$ 0.0	$ 0.0	$ 0.0
Contingent obligation | Level 3 | Minimum
Purchases, sales, settlements
Open position to offset fixed prices | MT		14,000	14,000
Contingent obligation | Level 3 | Maximum
Purchases, sales, settlements
Open position to offset fixed prices | MT		15,000	15,000

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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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