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Debt - Contingent obligation (Details)
$ in Millions
12 Months Ended
Apr. 29, 2019
USD ($)
installment
Dec. 31, 2019
USD ($)
payment
Dec. 31, 2018
USD ($)
Long-term Debt [Member] | E.ON Contingent Obligation [Member]      
Debt Instrument [Line Items]      
Number of monthly installments | payment   72  
Stated interest rate   10.94%  
Realized gain (loss) on contingent obligation   $ 1.4 $ 1.4
Other Liabilities [Member] | E.ON Contingent Obligation [Member]      
Debt Instrument [Line Items]      
Contingent obligation – principal   (12.9) (12.9)
Contingent obligation – accrued interest   (12.3) (10.9)
Contingent obligation – derivative asset   25.2 23.8
Contingent obligation   $ 0.0 $ 0.0
Hawesville [Member]      
Debt Instrument [Line Items]      
Due to Related Parties $ 40.0    
Number of monthly installments | installment 24    
London Interbank Offered Rate (LIBOR) [Member] | Hawesville [Member]      
Debt Instrument [Line Items]      
Stated interest rate, percentage 5.375%