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Derivatives - Narrative (Details) € in Millions, $ in Millions	6 Months Ended
	Jun. 30, 2022 USD ($) MWh t	Jun. 30, 2022 EUR (€)	Dec. 31, 2021 USD ($)
Derivative [Line Items]
Restricted cash \| $	$ 1.3		$ 8.6
LME Swap [Member]
Derivative [Line Items]
Open position to offset fixed prices (in tonnes) \| t	109,566
Midwest Premium (MWP) [Member]
Derivative [Line Items]
Open position to offset fixed prices (in tonnes) \| t	97,500
Fixed For Floating Swaps [Member]
Derivative [Line Items]
Open position to offset fixed prices (in tonnes) \| t	3,277
Nordpool Power Price Swap [Member] \| Grundartangi [Member]
Derivative [Line Items]
Derivative liability (in MwH) \| MWh	1,536,700
FX Swap [Member]
Derivative [Line Items]
Derivative asset \| €		€ 43.1
Indiana Hub Power Price Swaps [Member]
Derivative [Line Items]
Derivative liability (in MwH) \| MWh	263,520
USD ISK Forward Swap [Member] \| Not designated as hedging instrument [Member] \| Short [Member]
Derivative [Line Items]
Derivative, forward contracts \| $	$ 58.0
USD Euro Forward Swap [Member] \| Not designated as hedging instrument [Member] \| Long [Member]
Derivative [Line Items]
Derivative, forward contracts \| $	$ 14.8