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Business combinations - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended
Nov. 01, 2016
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Business Acquisition [Line Items]        
Operating income (loss)   $ (3.6) $ (2.1)  
Real Alloy        
Business Acquisition [Line Items]        
Accounts receivable due from Beck Trading   3.3   $ 6.8
Trade payables due to Beck Trading   0.1   $ 0.5
Beck Aluminum International, LLC (“Beck Trading”)        
Business Acquisition [Line Items]        
Percentage of voting interests acquired 49.00%      
Cash paid at closing $ 23.6      
Inventories 10.6      
Fair value adjustment to inventory $ 0.3      
Distributions received   6.0    
Operating income (loss)   $ 1.1