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Business acquisitions - Acquisition of Mt. Holly smelter narrative (Details) - USD ($)
$ in Thousands
 1 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
Dec. 01, 2014
Apr. 30, 2015
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Dec. 31, 2014
Business Acquisition [Line Items]              
Unrealized gain on fair value of contingent consideration     $ 10,287 $ 0 $ 16,814 $ 0  
Mt. Holly Smelter [Member]              
Business Acquisition [Line Items]              
Purchase price $ 67,500            
Purchase price cash paid   $ 38,162         $ 53,831
Acquisition costs     417   417   1,087
Contingent consideration, high 22,500            
Contingent consideration, low 12,500            
Contingent consideration at acquisition date 13,780            
Unrealized gain on fair value of contingent consideration     $ 10,287   $ 16,814    
Consideration receivable, economic adjustment 11,189            
Consideration received for working capital adjustments 124            
Consideration received for expected future post-employment benefits 2,400            
Consideration receivable for tax reimbursement 200            
Pension funding total 34,595            
Pension funding for unfunded GAAP liability 3,753            
Pension funding for 414(I) liability $ 30,842            
Equity interest percentage, pre step acquisition 49.70%            
Remeasurement of existing equity investment $ 56,723            
Gain on remeasurement of equity investment             $ 1,318