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Derivatives - Narrative (Details) € in Millions, $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2022 USD ($)	Sep. 30, 2022 USD ($) MWh t	Sep. 30, 2022 EUR (€)	Dec. 31, 2021 USD ($)
Derivative [Line Items]
Restricted cash	$ 0.0	$ 0.0		$ 8.6
LME Swap [Member]
Derivative [Line Items]
Open position to offset fixed prices (in tonnes) \| t		148,416
Midwest Premium (MWP) [Member]
Derivative [Line Items]
Open position to offset fixed prices (in tonnes) \| t		48,750
Fixed For Floating Swaps [Member]
Derivative [Line Items]
Open position to offset fixed prices (in tonnes) \| t		1,014
Nordpool Power Price Swap [Member] \| Grundartangi [Member]
Derivative [Line Items]
Derivative liability (in MwH) \| MWh		1,239,497
Realized gain	16.0	$ 16.0
FX Swap [Member]
Derivative [Line Items]
Derivative asset \| €			€ 37.9
Indiana Hub Power Price Swaps [Member]
Derivative [Line Items]
Derivative liability (in MwH) \| MWh		219,360
USD ISK Forward Swap [Member] \| Not designated as hedging instrument [Member] \| Short [Member]
Derivative [Line Items]
Derivative, forward contracts	52.0	$ 52.0
USD Euro Forward Swap [Member] \| Not designated as hedging instrument [Member] \| Long [Member]
Derivative [Line Items]
Derivative, forward contracts	$ 13.8	$ 13.8