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Business acquisitions - Acquisition of Sebree aluminum smelter (Details)
$ / shares in Units, t in Thousands, $ in Thousands
 1 Months Ended 3 Months Ended 12 Months Ended
Dec. 01, 2014
USD ($)
employee
t
Jun. 01, 2013
USD ($)
employee
t
Jul. 31, 2014
USD ($)
Jun. 30, 2015
USD ($)
Mar. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Jun. 30, 2014
USD ($)
Mar. 31, 2014
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
$ / shares
Dec. 31, 2013
USD ($)
$ / shares
Business Acquisition [Line Items]                      
Purchase price                 $ 0 $ 1,042 $ 48,058
Unrealized gain on fair value of contingent consideration         $ 6,527 $ 7,943     18,337 7,943 0
Gain on bargain purchase                 0 0 5,253
Business Acquisition, Pro Forma Information [Abstract]                      
Sebree power contract amortization               $ 5,534 0 5,534 31,031
Mt. Holly Smelter [Member]                      
Business Acquisition [Line Items]                      
Primary aluminum capacity (in tonnes) | t 231                    
Employees | employee 600                    
Purchase price $ 67,500                    
Purchase price 67,500                    
Unrealized gain on fair value of contingent consideration       $ 10,287         $ 18,337    
Consideration:                      
Total consideration 67,767                    
Assets Acquired:                      
Inventories 23,979                    
Prepaid and other current assets 2,527                    
Property, plant and equipment – net 142,837                    
Total assets acquired 214,034                    
Liabilities Assumed:                      
Accrued and other current liabilities 8,590                    
Accrued pension benefit costs 34,595                    
Accrued postretirement benefit costs 2,857                    
Other liabilities 8,213                    
Deferred taxes 2,686                    
Total liabilities assumed $ 98,412                    
Business Acquisition, Pro Forma Information [Abstract]                      
Sebree revenue                   25,911  
Sebree income from continuing operations                   3,024  
Pro forma revenues                   2,176,552 1,707,838
Pro forma loss from continuing operations                   $ 125,847 $ (38,819)
Pro forma loss per common share, basic (in dollars per share) | $ / shares                   $ 1.30 $ (0.44)
Pro forma loss per common share, diluted (in dollars per share) | $ / shares                   $ 1.29 $ (0.44)
Sebree Smelter [Member]                      
Business Acquisition [Line Items]                      
Primary aluminum capacity (in tonnes) | t   218                  
Employees | employee   500                  
Purchase price   $ 61,000                  
Purchase price     $ 49,035                
Unrealized gain on fair value of contingent consideration             $ 965        
Gain on bargain purchase   5,253                 $ 5,253
Consideration:                      
Total consideration   48,083                  
Deferred purchase price   1,910                  
Assets Acquired:                      
Inventories   59,018                  
Prepaid and other current assets   2,273                  
Property, plant and equipment – net   55,520                  
Total assets acquired   116,811                  
Liabilities Assumed:                      
Accrued and other current liabilities   43,316                  
Accrued pension benefit costs   996                  
Accrued postretirement benefit costs   6,544                  
Other liabilities   7,476                  
Deferred taxes   3,233                  
Total liabilities assumed   $ 61,565                  
Business Acquisition, Pro Forma Information [Abstract]                      
Sebree revenue                     247,178
Sebree income from continuing operations                     8,705
Sebree power contract amortization                   $ 5,534 31,031
Pro forma revenues                     1,662,707
Pro forma loss from continuing operations                     $ (83,035)
Pro forma loss per common share, basic (in dollars per share) | $ / shares                     $ (0.94)
Pro forma loss per common share, diluted (in dollars per share) | $ / shares                     $ (0.94)