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Business acquisitions - Acquisition of Mt. Holly aluminum smelter narrative (Details)
t in Thousands, $ in Thousands
 1 Months Ended 3 Months Ended 12 Months Ended 13 Months Ended
Dec. 01, 2014
USD ($)
employee
t
Apr. 30, 2015
USD ($)
Dec. 31, 2014
USD ($)
Jun. 30, 2015
USD ($)
Mar. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Dec. 31, 2015
USD ($)
Business Acquisition [Line Items]                    
Gain on fair value of contingent consideration         $ 6,527 $ 7,943 $ 18,337 $ 7,943 $ 0  
Gain on remeasurement of equity investment           15,955 $ 0 15,955 $ 0  
Mt. Holly Smelter [Member]                    
Business Acquisition [Line Items]                    
Percentage of voting interests acquired 50.30%                  
Percentage of ownership after acquisition (percentage)             100.00%     100.00%
Employees | employee 600                  
Primary aluminum capacity (in tonnes) | t 231                  
Purchase price $ 67,500                  
Acquisition-related costs     $ 1,087     $ 1,087 $ 452 $ 1,087   $ 452
Contingent consideration, maximum 22,500                  
Contingent consideration arrangements 12,500                  
Contingent consideration 13,780                  
Gain on fair value of contingent consideration       $ 10,287     18,337      
Contingent consideration receivable             $ 12,500     12,500
Consideration receivable, economic adjustment   $ 11,189                
Consideration received for working capital adjustments   124                
Consideration received for expected future post-employment benefits $ 2,400                  
Equity interest in acquiree (percentage) 49.70%                  
Fair value of equity interest in acquiree $ 47,855                  
Gain on remeasurement of equity investment     $ 15,955             $ 14,637
Pension funding total   $ 34,595                
Due from Alumax, purchase price credit $ 11,269