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Fair Value Measurements - Significant Unobservable Inputs (Details) $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($) MWh T	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Locational margin price average term	3 years
LME Swap [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ 6.1	$ 7.8	$ 10.6
LME Swap [Member] | Discounted Cash Flow [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, percentage	0.1000
Contingent obligation [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ 0.0
Contingent obligation [Member] | Minimum [Member] | Discounted Cash Flow [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset (in tones) | T	1,594
Contingent obligation [Member] | Maximum [Member] | Discounted Cash Flow [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset (in tones) | T	2,179
Hawesville L4 Power Price Swaps [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ (1.5)	$ (3.2)	$ (2.9)	$ (2.7)	$ 0.0	$ 0.0
Hawesville L4 Power Price Swaps [Member] | Discounted Cash Flow [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability (in MwH) | MWh	0.52

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Value of input used to measure derivative asset.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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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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