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Investments in Mortgage Revenue Bonds - Additional Information (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Mar. 31, 2016
Schedule Of Available For Sale Securities [Line Items]        
Other income $ 828,089   $ 373,379  
Investment income 48,225,068 $ 36,892,996 34,409,809  
Contingent interest income $ 3,147,165 $ 2,021,077 $ 4,756,716  
San Antonio, Texas [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Percentage of available for sale securities by location 44.00% 45.00%    
California [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Percentage of available for sale securities by location 20.00% 20.00%    
South Carolina [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Percentage of available for sale securities by location 16.00% 12.00%    
Ashley Square [Member] | IA [Member] | Mortgage Revenue Bonds Held In Trust [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Contingent interest income $ 2,900,000      
Pro Nova 2014-1 [Member] | Term TOB Facility [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Debt financing       $ 8,400,000
Series B [Member] | Vantage at Harlingen [Member] | TX [Member] | Mortgage Revenue Bonds Held In Trust [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Other income 424,000      
Series B [Member] | Avistar at Chase Hill [Member] | TX [Member] | Mortgage Revenue Bonds Held In Trust [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Other income 200,000      
Investment income 101,000      
Series B [Member] | Heights at 515 [Member] | TX [Member] | Mortgage Revenue Bonds Held In Trust [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Principal Outstanding $ 510,000