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Business acquisitions - Acquisition of Mt. Holly smelter narrative (Details) - USD ($)
$ in Thousands
 1 Months Ended 3 Months Ended 9 Months Ended 10 Months Ended
Dec. 01, 2014
Apr. 30, 2015
Dec. 31, 2014
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Business Acquisition [Line Items]                
Unrealized gain on fair value of contingent consideration       $ 1,523 $ 0 $ 18,337 $ 0  
Mt. Holly Smelter [Member]                
Business Acquisition [Line Items]                
Purchase price $ 67,500              
Acquisition costs     $ 1,087 417   417   $ 417
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       $ 1,523   $ 18,337    
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 percentage, pre step acquisition 49.70%              
Remeasurement of existing equity investment $ 47,700              
Gain on remeasurement of equity investment     $ 15,956         $ 14,638
Consideration transferred $ 67,767              
Pension funding total   $ 34,595