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Credit Facility - Additional Information (Detail) - USD ($)
Dec. 08, 2016
Apr. 08, 2018
Mar. 31, 2018
Dec. 31, 2017
Line of Credit Facility [Line Items]        
Debt fee     $ 668,000 $ 1,372,000
Senior Secured Revolving Credit Facility        
Line of Credit Facility [Line Items]        
Revolving credit facility     16,000,000 $ 0
Undrawn availability     $ 29,000,000  
Senior Secured Revolving Credit Facility | Jeffries Finance LLC        
Line of Credit Facility [Line Items]        
Line of credit facility, maximum borrowing capacity $ 45,000,000      
Credit facility agreement, term 3 years      
Subsequent event | Senior Secured Revolving Credit Facility | Jeffries Finance LLC        
Line of Credit Facility [Line Items]        
Line of credit facility, maximum borrowing capacity   $ 60,000,000