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Business acquisitions - Acquisition of Mt. Holly aluminum smelter (Details) - Mt. Holly Smelter [Member] - USD ($)
$ / shares in Units, $ in Thousands
 10 Months Ended 12 Months Ended
Sep. 30, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 01, 2014
Assets Acquired:        
Inventories       $ 23,979
Due from Alumax       11,269
Prepaid and other current assets       2,527
Intangible asset       2,580
Pension asset       30,842
Property, plant and equipment – net       142,837
Total assets acquired       214,034
Liabilities Assumed:        
Accounts payable, trade       41,471
Accrued and other current liabilities       8,590
Accrued pension benefit costs       34,595
Accrued postretirement benefit costs       2,857
Asset retirement obligations       8,213
Deferred taxes       2,686
Total liabilities assumed       98,412
Goodwill       0
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract]        
Inventories $ (2,126)      
Due from Alumax (9,517)      
Property, plant and equipment – net 15,748      
Total assets acquired 4,105      
Accrued and other current liabilities 255      
Accrued pension benefit costs 34,595      
Deferred taxes (2,118)      
Total liabilities assumed 32,732      
Goodwill $ (4,804)      
Business Acquisition, Pro Forma Information [Abstract]        
Mt. Holly revenue   $ 25,911    
Mt. Holly income from continuing operations   3,024    
Pro forma revenues   2,176,552 $ 1,707,838  
Pro forma earnings from continuing operations   $ 125,847 $ (38,819)  
Pro forma earnings per common share, basic (in dollars per share)   $ 1.30 $ (0.44)  
Pro forma earnings per common share, diluted (in dollars per share)   $ 1.29 $ (0.44)  
Preliminary estimate [Member]        
Assets Acquired:        
Inventories       26,105
Due from Alumax       20,786
Prepaid and other current assets       2,527
Intangible asset       2,580
Pension asset       30,842
Property, plant and equipment – net       127,089
Total assets acquired       209,929
Liabilities Assumed:        
Accounts payable, trade       41,471
Accrued and other current liabilities       8,335
Accrued pension benefit costs       0
Accrued postretirement benefit costs       2,857
Asset retirement obligations       8,213
Deferred taxes       4,804
Total liabilities assumed       65,680
Goodwill       $ 4,804