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Fair Value Measurements - Level 3 Reconciliation (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2020 | Mar. 31, 2019
LME Swap [Member] |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward] |  |
Balance, beginning | $ 10,600 |
Total realized/unrealized gains (losses) |  |
Included in net income (loss) | 6,700 |
Purchases, sales, settlements |  |
Purchases | 0 |
Sales | 0 |
Settlements | 0 |
Transfers into Level 3 | 0 |
Transfers out of Level 3 | (9,500) |
Balance, ending | 7,800 |
Change in unrealized gains (losses) | 6,700 |
Hawesville L4 Power Price Swaps [Member] |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward] |  |
Balance, beginning | (2,900) |
Total realized/unrealized gains (losses) |  |
Included in net income (loss) | (1,500) |
Purchases, sales, settlements |  |
Purchases | 0 |
Sales | 0 |
Settlements | 1,200 |
Transfers into Level 3 | 0 |
Transfers out of Level 3 | 0 |
Balance, ending | (3,200) |
Change in unrealized gains (losses) | $ (300) |
Nordpool Power Price Swap [Member] |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward] |  |
Balance, beginning |  | $ 4,700
Total realized/unrealized gains (losses) |  |
Included in net income (loss) |  | (1,000)
Purchases, sales, settlements |  |
Purchases |  | 0
Sales |  | 0
Settlements |  | 0
Transfers into Level 3 |  | 0
Transfers out of Level 3 |  | 0
Balance, ending |  | 3,700
Change in unrealized gains (losses) |  | (1,000)
Midwest Premium (MWP) [Member] |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward] |  |
Balance, beginning |  | 0
Total realized/unrealized gains (losses) |  |
Included in net income (loss) |  | (1,700)
Purchases, sales, settlements |  |
Purchases |  | 0
Sales |  | 0
Settlements |  | 200
Transfers into Level 3 |  | 600
Transfers out of Level 3 |  | 0
Balance, ending |  | (900)
Change in unrealized gains (losses) |  | (900)
FX Swap [Member] |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward] |  |
Balance, beginning |  | (300)
Total realized/unrealized gains (losses) |  |
Included in net income (loss) |  | (300)
Purchases, sales, settlements |  |
Purchases |  | 0
Sales |  | 0
Settlements |  | 0
Transfers into Level 3 |  | 0
Transfers out of Level 3 |  | 0
Balance, ending |  | (600)
Change in unrealized gains (losses) |  | (300)
United States [Member] | LME Swap [Member] |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward] |  |
Balance, beginning |  | 300
Total realized/unrealized gains (losses) |  |
Included in net income (loss) |  | (1,900)
Purchases, sales, settlements |  |
Purchases |  | 0
Sales |  | 0
Settlements |  | 0
Transfers into Level 3 |  | (200)
Transfers out of Level 3 |  | 0
Balance, ending |  | (1,800)
Change in unrealized gains (losses) |  | (2,100)
Iceland [Member] | LME Swap [Member] |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward] |  |
Balance, beginning |  | (200)
Total realized/unrealized gains (losses) |  |
Included in net income (loss) |  | (100)
Purchases, sales, settlements |  |
Purchases |  | 0
Sales |  | 0
Settlements |  | 0
Transfers into Level 3 |  | 0
Transfers out of Level 3 |  | 0
Balance, ending |  | (300)
Change in unrealized gains (losses) |  | $ (100)

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- Definition 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). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19279-110258 + Details
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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19279-110258 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19279-110258 + Details
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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19279-110258 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19279-110258 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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