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Business combinations - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Nov. 01, 2016
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Business Acquisition [Line Items]            
Operating income (loss)   $ 3.8 $ 8.1 $ 0.2 $ 6.0  
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)       0.6    
Beck Trading | Real Alloy | Accounts Receivable            
Business Acquisition [Line Items]            
Due from (to) related party   1.7   1.7   $ 6.8
Beck Trading | Real Alloy | Trade Payables            
Business Acquisition [Line Items]            
Due from (to) related party   $ (0.1)   $ (0.1)   $ (0.5)