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Fair Value Measurements - Level 3 Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Nord Pool Swap [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance, beginning	$ 0.0		$ 0.0
Total realized/unrealized gains (losses)
Included in net income (loss)	0.0		0.0
Purchases, sales, settlements
Purchases	0.0		0.0
Sales	0.0		0.0
Settlements	0.0		0.0
Transfers into Level 3	1.5		1.5
Transfers out of Level 3	0.0		0.0
Balance, ending	1.5		1.5
Change in unrealized gains (losses)	0.0		0.0
LME Swap [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance, beginning	(6.0)	$ 7.8	2.9	$ 10.6
Total realized/unrealized gains (losses)
Included in net income (loss)	(11.6)	(1.7)	(20.3)	5.0
Purchases, sales, settlements
Purchases	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Transfers into Level 3	(0.3)	0.0	(1.0)	0.0
Transfers out of Level 3	0.0	0.0	0.5	(9.5)
Balance, ending	(17.9)	6.1	(17.9)	6.1
Change in unrealized gains (losses)	(11.6)	(1.7)	(20.3)	5.0
FX Swap [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance, beginning	(0.2)		0.1
Total realized/unrealized gains (losses)
Included in net income (loss)	0.0		0.0
Purchases, sales, settlements
Purchases	0.0		0.0
Sales	0.0		0.0
Settlements	0.0		0.0
Transfers into Level 3	(0.2)		(0.4)
Transfers out of Level 3	0.0		(0.1)
Balance, ending	(0.4)		(0.4)
Change in unrealized gains (losses)	$ 0.0		$ 0.0
Hawesville L4 Power Price Swaps [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance, beginning		(3.2)		(2.9)
Total realized/unrealized gains (losses)
Included in net income (loss)		0.0		(1.4)
Purchases, sales, settlements
Purchases		0.0		0.0
Sales		0.0		0.0
Settlements		1.7		2.8
Transfers into Level 3		0.0		0.0
Transfers out of Level 3		0.0		0.0
Balance, ending		(1.5)		(1.5)
Change in unrealized gains (losses)		$ 1.7		$ 1.4

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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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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 settlements of financial instrument classified as a derivative asset (liability) after deduction of derivative liability, 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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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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