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Quarterly information (Unaudited) - Financial Results Overview (Details) - USD ($)
 3 Months Ended 12 Months Ended
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Effect of Fourth Quarter Events [Line Items]                      
Helguvik impairment                 $ 152,220,000 $ 0 $ 0
Charge related to discrete tax items $ 6,900,000                    
Impairment charge                 3,830,000 30,850,000 0
Ravenswood retiree legal settlement   $ 23,000,000             23,000,000 0 0
Lower cost or market inventory adjustment           $ 5,324,000     (660,000) 7,539,000 (1,247,000)
Favorable lower of cost or market inventory adjustment       $ 5,800,000 $ 23,474,000            
Pension charges         3,400,000            
Depreciation                 84,780,000 80,117,000 70,827,000
Impairment                 0 11,584,000 0
Labor disruption costs           1,400,000 $ 11,700,000        
Gain on fair value of contingent consideration               $ 6,527,000 0 $ 18,337,000 $ 7,943,000
Severance charge               1,000,000      
Hawesville and Mt. Holly [Member]                      
Effect of Fourth Quarter Events [Line Items]                      
Curtailment expenses         3,500,000            
Hawesville [Member]                      
Effect of Fourth Quarter Events [Line Items]                      
Curtailment expenses           $ 2,900,000          
Ravenswood Aluminum Smelter [Member]                      
Effect of Fourth Quarter Events [Line Items]                      
Impairment charge   $ 3,800,000         30,850,000   $ 3,830,000    
Lower cost or market inventory adjustment     $ 4,900,000       25,689,000        
Grundartangi, Iceland [Member]                      
Effect of Fourth Quarter Events [Line Items]                      
Signing bonuses, amount               $ 1,570,000      
Mt. Holly Smelter [Member]                      
Effect of Fourth Quarter Events [Line Items]                      
Depreciation         5,000,000            
Gain on fair value of contingent consideration             $ 10,287,000        
BHH [Member]                      
Effect of Fourth Quarter Events [Line Items]                      
Impairment         $ 11,584,000            
Helguvik [Member]                      
Effect of Fourth Quarter Events [Line Items]                      
Helguvik impairment $ 152,200,000