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Fair Value Measurements - Significant Unobservable Inputs (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) MT t	Dec. 31, 2020 USD ($) MT	Dec. 31, 2019 USD ($)
Nordpool Power Price Swap
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ 0.2	$ 0.0
Nordpool Power Price Swap | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ 0.2	0.0
Derivative asset, percentage	0.0858
LME Swap
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ (5.1)	$ 2.9	$ 10.6
Open position to offset fixed prices | t	123,216
LME Swap | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, percentage	0.0858
LME Swap | Discounted Cash Flow | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, percentage		0.1000
FX Swap
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ (0.2)	$ 0.1	$ 0.0
FX Swap | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, percentage	0.0858
FX Swap | Discounted Cash Flow | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, percentage		0.1000
Contingent obligation
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ 0.0	$ 0.0
Contingent obligation | Level 3 | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Open position to offset fixed prices | MT	14,000	14,000
Contingent obligation | Level 3 | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Open position to offset fixed prices | MT	15,000	15,000

X

- Definition

Value of input used to measure derivative asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

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- Definition

Nominal mass used to calculate payments on a derivative instrument.

+ References

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+ References

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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19279-110258

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