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Summary of Significant Accounting Policies - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended
Sep. 30, 2015
Dec. 31, 2017
Dec. 31, 2016
Sep. 30, 2017
Government Contracts Concentration Risk | Sales Revenue | Surplus Contract        
Business Combination [Line Items]        
Concentration risk (as a percentage)   26.50% 29.90%  
Government Contracts Concentration Risk | Sales Revenue | Scrap Contract        
Business Combination [Line Items]        
Concentration risk (as a percentage)   8.40% 10.00%  
Supplier Concentration Risk | Cost of Goods | Contract With Commercial Client        
Business Combination [Line Items]        
Concentration risk (as a percentage)   19.40% 17.80%  
Tanager Acquisitions Promissory Note        
Business Combination [Line Items]        
Receivable with imputed interest, face amount $ 12.3      
Receivable with imputed interest, term 5 years      
Proceeds from collection of notes receivable $ 2.5      
Receivable with imputed interest, net amount   $ 9.8    
Tanager Acquisitions Promissory Note | Other Assets        
Business Combination [Line Items]        
Receivable with imputed interest, net amount   8.3    
Tanager Acquisitions Promissory Note | Prepaid Expenses and Other Current Assets        
Business Combination [Line Items]        
Receivable with imputed interest, net amount   1.5    
Accounting Standards Update 2016-09 | Retained Earnings        
Business Combination [Line Items]        
Cumulative effect of new accounting principle in period of adoption   $ (0.2)   $ (0.2)