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Business acquisitions - Acquisition of Mt. Holly smelter narrative (Details) - USD ($)
$ in Thousands
 1 Months Ended 3 Months Ended 12 Months Ended
Dec. 01, 2014
Mar. 31, 2016
Apr. 30, 2015
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Dec. 31, 2014
Business Acquisition [Line Items]              
Unrealized gain on fair value of contingent consideration       $ 0 $ 6,527    
Total net cash consideration paid $ 41,487            
Mt. Holly Smelter [Member]              
Business Acquisition [Line Items]              
Purchase price 67,500            
Acquisition costs 1,539            
Contingent consideration at acquisition date 13,780            
Unrealized gain on fair value of contingent consideration       $ 6,527   $ 18,337 $ 7,943
Proceeds from acquisition - contingent consideration   $ 12,500          
Consideration receivable, economic adjustment     $ 11,189        
Consideration received for working capital adjustments     $ 124        
Consideration due for expected future post-employment benefits 200            
Consideration received for expected future post-employment benefits $ 2,400