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Long-Term Debt (Details) - USD ($)
Apr. 17, 2017
Jun. 30, 2018
Jan. 09, 2018
Dec. 31, 2017
Debt instrument        
Long-term debt   $ 0   $ 0
Letters of credit outstanding   0    
Letter of Credit        
Debt instrument        
Line of credit   $ 2,800,000    
Texas Capital Credit Facility        
Debt instrument        
Borrowing capacity $ 20,000,000   $ 22,000,000  
Sublimit available for letters of credit $ 5,000,000      
Unused capacity commitment fee percentage (as percent) 0.50%      
Quarterly collateral fees $ 100,000      
Texas Capital Credit Facility | Variable rate one        
Debt instrument        
Variable rate basis LIBOR      
Rate spread on variable rate (as percent) 1.95%      
Texas Capital Credit Facility | Variable rate two        
Debt instrument        
Variable rate basis prime      
Rate spread on variable rate (as percent) 0.75%