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Share-based Compensation - Narrative (Details)
$ / shares in Units, shares in Thousands, $ in Thousands
6 Months Ended
Apr. 25, 2018
$ / shares
shares
Sep. 01, 2017
shares
$ / shares
Jun. 30, 2018
USD ($)
$ / shares
Dec. 31, 2017
$ / shares
Restricted Stock        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Unrecognized compensation expense     $ 4,184  
Weighted average recognition period     2 years 1 month 3 days  
A.M. Castle & Co. 2017 Management Incentive Plan | Restricted Stock        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares convertible (in shares) | shares   638    
Award vesting period 1 year 3 years    
Weighted average grant date fair value (in dollars per share) | $ / shares   $ 3.14 $ 3.18 $ 3.14
Granted (in dollars per share) | $ / shares $ 4.35   $ 4.35  
Restricted shares issued (in shares) | shares 69 1,734    
Second Lien Notes | A.M. Castle & Co. 2017 Management Incentive Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Convertible debt     $ 2,400  
Unrecognized convertible debt compensation expense     $ 1,661  
Weighted average recognition period     2 years 2 months  
Vesting period     3 years  
Fair value of debt     $ 2,300