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Debt - Credit Facility (Details) |  |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Jul. 01, 2020 USD ($) | Apr. 29, 2019 USD ($) installment | Sep. 30, 2020 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Jun. 30, 2020 USD ($)
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Proceeds from issuance of debt |  |  |  | $ 243,800,000 | $ 0 | $ 0 |
Loss on early extinguishment of debt |  |  |  | (1,200,000) | $ 0 | $ 0 |
U.S revolving credit facility [Member] |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Credit facility maximum amount |  |  |  | 175,000,000.0 |  |  |
Borrowing availability |  |  |  | 129,900,000 |  |  |
Outstanding letters of credit issued |  |  |  | 34,300,000 |  |  |
Letter of Credit [Member] |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Letter of credit sub-facility amount |  |  |  | 95,600,000 |  |  |
Short-term Debt [Member] | Letter of Credit [Member] |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Outstanding borrowings |  |  |  | 0 |  |  |
Short-term Debt [Member] | Iceland revolving credit facility [Member] |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Outstanding borrowings |  |  |  | 45,000,000.0 |  |  |
Hawesville [Member] |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Due to related parties |  | $ 40,000,000.0 |  |  |  |  |
Number of monthly payments | installment |  | 24 |  |  |  |  |
Hawesville [Member] | London Interbank Offered Rate (LIBOR) [Member] |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate |  | 5.375% |  |  |  |  |
Senior secured notes due June 01, 2025 [Member] |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Proceeds from issuance of debt | $ 243,800,000 |  |  |  |  |  |
Senior secured notes due June 01, 2025 [Member] | Level 2 |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Fair value of debt instrument |  |  |  | 265,000,000.0 |  |  |
Long-term Debt [Member] | U.S revolving credit facility [Member] |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Credit facility face amount |  |  |  | 175,000,000.0 |  |  |
Long-term Debt [Member] | Letter of Credit [Member] |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Letter of credit sub-facility amount |  |  |  | 110,000,000.0 |  |  |
Long-term Debt [Member] | U.S revolving credit facility, accordion [Member] |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Credit facility face amount |  |  |  | 50,000,000.0 |  |  |
Long-term Debt [Member] | Iceland revolving credit facility [Member] |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Credit facility maximum amount |  |  |  | 50,000,000.0 |  |  |
Borrowing availability |  |  |  | 5,000,000.0 |  |  |
Outstanding letters of credit issued |  |  |  | 0 |  |  |
Borrowing availability, net of outstanding letters of credit and borrowings |  |  |  | $ 5,000,000.0 |  |  |
Long-term Debt [Member] | Senior secured notes due June 01, 2025 [Member] |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Stated interest rate |  |  |  | 12.00% |  |  |
Face amount | $ 250,000,000.0 |  |  |  |  |  |
Interest rate, cash percentage | 10.00% |  |  |  |  |  |
Interest rate, additional notes or cash percentage | 2.00% |  |  |  |  |  |
Redemption price, percentage |  |  |  | 100.00% |  |  |
Long-term Debt [Member] | Senior secured notes due June 01, 2021 [Member] |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Stated interest rate |  |  |  | 7.50% |  |  | 7.50%
Face amount |  |  |  |  |  |  | $ 243,700,000
Redemption price, percentage |  |  |  | 100.00% |  |  |
Loss on early extinguishment of debt |  |  | $ 1,200,000 | $ 1,200,000 |  |  |

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- Definition Debt Instrument, Payment Terms, Number of Monthly Payments + References No definition available. + Details
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- Definition The amount of additional borrowings provided in credit facility under a letter of credit sub-facility. + References No definition available. + Details
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- Definition Line of Credit Facility, Remaining Borrowing Capacity, Net of Letters of Credit + References No definition available. + Details
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- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
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- Definition Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611 + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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- Definition Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 50 -Section 40 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=117329964&loc=d3e12355-112629 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 50 -Section 40 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=117329964&loc=d3e12317-112629 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 + Details
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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910 + Details
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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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- Definition The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3255-108585 + Details
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