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Debt - Credit Facility (Details) | Apr. 29, 2019 USD ($) payment | Jun. 30, 2019 USD ($) | Dec. 31, 2018 USD ($) | Jun. 04, 2013 USD ($)
--- | --- | --- | --- | ---
Line of Credit Facility [Line Items] |  |  |  |
Fair value of debt instrument |  | $ 249,000,000.0 | $ 247,900,000 |
U.S. revolving credit facility [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Credit facility maximum amount |  | 175,000,000.0 |  |
Borrowing availability |  | 172,300,000 |  |
Borrowing availability, net of outstanding letters of credit and borrowings |  | 37,600,000 |  |
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] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Letter of credit sub-facility amount |  | 126,500,000 |  |
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] |  |  |  |
Credit facility maximum amount |  | 50,000,000.0 |  |
Borrowing availability |  | 50,000,000.0 |  |
Borrowing availability, net of outstanding letters of credit and borrowings |  | 0 |  |
Borrowing availability, net of borrowings |  | $ 50,000,000.0 |  |
Senior secured notes due June 01, 2021 [Member] | Long-term Debt [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Face amount |  |  |  | $ 250,000,000.0
Stated interest rate, percentage |  | 7.50% |  | 7.50%
Short-term Debt [Member] | U.S. letter of credit [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Outstanding borrowings |  | $ 8,100,000 |  |
Short-term Debt [Member] | Iceland revolving line of credit [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Outstanding borrowings |  | $ 0 |  |
Short-term Debt [Member] | U.S. revolving credit facility borrowings [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Stated interest rate, percentage |  | 5.375% |  |
U.S revolving credit facility |  | $ 8,100,000 | $ 23,300,000 |
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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- 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 Face (par) amount of debt instrument at time of issuance. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400 + 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=109250915&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=6877327&loc=d3e13212-122682 + Details
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- Definition Carrying amount as of the balance sheet date of obligations due all related parties. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(k)(1)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.17) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.15(3),(4)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + 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=6877327&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=6876686&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=6879938&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=6877327&loc=d3e13212-122682 + 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=6877327&loc=d3e13212-122682 + Details
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- Definition Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding). + 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=6877327&loc=d3e13212-122682 + Details
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- Definition Carrying value as of the balance sheet date of the portion of long-term, collateralized debt obligations due within one year or the operating cycle, if longer. Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower. + References Reference 1: 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(a)(16)(a)(1)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 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(13)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: 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)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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