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Long-Term Debt - Senior Secured Credit Facility - Additional Information (Details) - USD ($)
1 Months Ended 3 Months Ended
Jul. 31, 2015
Mar. 31, 2018
Jan. 04, 2018
Dec. 31, 2017
Debt Instrument [Line Items]        
Senior Secured Credit Facility   $ 73,000,000   $ 142,080,000
Senior Secured Credit Facility        
Debt Instrument [Line Items]        
Senior Secured Credit Facility   $ 73,000,000    
Weighted average interest rate on borrowings   4.84%    
Senior secured credit facility sub limit   $ 2,500,000    
Debt instrument borrowing base     $ 160,000,000  
Senior Secured Credit Facility | Minimum        
Debt Instrument [Line Items]        
Commitment fee percentage   0.375%    
Senior Secured Credit Facility | Maximum        
Debt Instrument [Line Items]        
Commitment fee percentage   0.50%    
Citibank N A | Senior Secured Credit Facility        
Debt Instrument [Line Items]        
Debt instrument maturity date Jul. 29, 2020      
LRAI | Citibank N A | Senior Secured Credit Facility        
Debt Instrument [Line Items]        
Debt instrument face amount $ 500,000,000