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Related party transactions - Narrative (Details)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2015
USD ($)
Dec. 31, 2016
USD ($)
t
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Related Party Transaction [Line Items]            
Glencore beneficial ownership       100.00%    
Major customer, percentage revenue, net       10.00%    
BHH impairment | $       $ 0 $ 11,584 $ 0
Glencore [Member]            
Related Party Transaction [Line Items]            
Glencore beneficial ownership 47.50%     47.50%    
BHH [Member]            
Related Party Transaction [Line Items]            
Ownership percentage, equity method 40.00%     40.00%    
BHH impairment | $     $ 11,584      
Glencore [Member]            
Related Party Transaction [Line Items]            
Other forward delivery contracts to sell primary aluminum (in tonnes) | t       60,600    
Customer Concentration Risk [Member] | Sales Revenue, Goods, Net [Member] | Glencore [Member]            
Related Party Transaction [Line Items]            
Major customer, percentage revenue, net 77.00% 88.00%   89.00%    
Commodity Contract [Member]            
Related Party Transaction [Line Items]            
Open position to offset fixed prices | t       9,531    
Common stock [Member] | Glencore [Member]            
Related Party Transaction [Line Items]            
Glencore beneficial ownership 42.90%     42.90%