Commitments and Contingencies - Other Commitments (Details)	12 Months Ended
	Dec. 31, 2019 labor_union MW
Labor Commitments [Abstract]
Percentage of Company's work force represented by a union	65.00%
Percentage of Grundartangi work force represented by the labor unions	86.00%
Number of labor unions under new agreement | labor_union	5
Percentage of U.S. based work force represented by a union	56.00%
Sebree [Member]
Loss Contingencies [Line Items]
Power supply agreement, termination notice period	1 year
South Carolina Public Service Authority [Member]
Loss Contingencies [Line Items]
Short-term power agreement, supply from South Carolina Public Service Authority (percent)	25.00%
Short-term power agreement, supply from third party supplier (percent)	75.00%
South Carolina Public Service Authority [Member] | Santee Cooper [Member]
Loss Contingencies [Line Items]
Power supply agreement, termination notice period	120 days
South Carolina Public Service Authority [Member] | Other Power Suppliers [Member]
Loss Contingencies [Line Items]
Power supply agreement, termination notice period	60 days
Grundartangi - HS, Landsvirkjun and OR [Member]
Loss Contingencies [Line Items]
Power available	525
Grundartangi - Landsvirkjun [Member]
Loss Contingencies [Line Items]
Power available	161
Helguvik [Member]
Loss Contingencies [Line Items]
Power available	47.5
Netherlands [Member]
Labor Commitments [Abstract]
Percentage of Vlissingen work force represented by the labor union	100.00%