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Derivatives (Details)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2017
USD ($)
MWh
t
Mar. 31, 2016
USD ($)
Dec. 31, 2016
USD ($)
t
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Derivative [Line Items]          
Derivative asset | $ $ 1,610        
Derivative liability | $ 14,915        
Recognized loss on derivative instruments | $ $ 16,137 $ (353) $ (3,487) $ (1,600) $ (179)
LME Swap [Member]          
Derivative [Line Items]          
Other forward delivery contracts to sell primary aluminum (in tonnes) 52,590        
Other forward delivery contracts to sell primary aluminum, settled monthly (in tonnes) 49,500        
Price Swap [Member]          
Derivative [Line Items]          
Open position to offset fixed prices 7,931        
Glencore [Member]          
Derivative [Line Items]          
Other forward delivery contracts to sell primary aluminum (in tonnes)     60,600    
Glencore [Member] | LME Swap [Member]          
Derivative [Line Items]          
Other forward delivery contracts to sell primary aluminum (in tonnes) 48,540        
Grundartangi [Member] | Power Price Swap [Member]          
Derivative [Line Items]          
Percentage of power purchases included in forward price contract (percent) 4.00%        
Power available | MWh 256,200