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Debt - Narrative (Details)	Apr. 09, 2021 USD ($)	Jul. 01, 2020 USD ($)	Apr. 29, 2019 USD ($) payment	Apr. 14, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
U.S. revolving credit facility [Member]
Line of Credit Facility [Line Items]
Outstanding borrowings					$ 50,400,000
Credit facility maximum amount					175,000,000.0
U.S. revolving credit facility [Member] | Long-term Debt [Member]
Line of Credit Facility [Line Items]
Credit facility face amount					175,000,000.0
U.S. letter of credit [Member] | Long-term Debt [Member]
Line of Credit Facility [Line Items]
Letter of credit sub-facility amount					110,000,000.0
U.S. revolving credit facility, accordion [Member] | Long-term Debt [Member]
Line of Credit Facility [Line Items]
Credit facility face amount					50,000,000.0
Iceland revolving line of credit [Member] | Long-term Debt [Member]
Line of Credit Facility [Line Items]
Outstanding borrowings					0
Credit facility maximum amount					50,000,000.0
Senior Secured Notes Due June 01, 2025 [Member]
Line of Credit Facility [Line Items]
Proceeds from issuance of debt		$ 243,800,000
Debt instrument, tender premium					14,600,000
Senior Secured Notes Due June 01, 2025 [Member] | Level 2 [Member]
Line of Credit Facility [Line Items]
Fair value of debt instrument					$ 268,800,000
Senior Secured Notes Due June 01, 2025 [Member] | Long-term Debt [Member]
Line of Credit Facility [Line Items]
Stated interest rate, percentage		12.00%			12.00%
Face amount		$ 250,000,000.0
Senior Secured Notes Due June 01, 2025 [Member] | Long-term Debt [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Face amount				$ 250,000,000.0
Debt instrument, tender premium				$ 14,600,000
Interest rate, cash percentage				7.50%
Convertible Senior Note Due 2028 [Member] | Long-term Debt [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Face amount	$ 86,300,000
Interest rate, cash percentage	2.75%
Percentage of aggregate principal amount issued	100.00%
Revolving Credit Facility [Member] | Iceland revolving line of credit [Member] | Long-term Debt [Member]
Line of Credit Facility [Line Items]
Revolving credit facility					$ 0	$ 45,000,000.0
Senior Secured Notes Due 2028 [Member] | Long-term Debt [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Stated interest rate, percentage				7.50%
Face amount				$ 250,000,000.0
Debt instrument, repurchased face amount				$ 195,900,000
Hawesville [Member]
Line of Credit Facility [Line Items]
Due to related parties, noncurrent			$ 40,000,000.0
Debt instrument, payment terms, number of monthly payments | payment			24
London Interbank Offered Rate (LIBOR) [Member] | Hawesville [Member]
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate			5.375%

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Debt Instrument, Interest Rate, Stated Percentage, Cash

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Debt Instrument, Payment Terms, Number of Monthly Payments

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The amount of additional borrowings provided in credit facility under a letter of credit sub-facility.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Face (par) amount of debt instrument at time of issuance.

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Face (par) amount of the original debt instrument that was repurchased.

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Carrying amount as of the balance sheet date of obligations due all related parties.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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