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Fair Value Measurements - Level 3 Reconciliation (Details) - USD ($)
$ in Thousands
3 Months Ended
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)