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Investments (Details) - USD ($)
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Dec. 31, 2015
Cash        
Cash $ 6,110,170 $ 6,249,703    
Cash equivalents        
Cash and cash equivalents at carrying value 47,178,505 31,228,192 $ 34,732,782 $ 15,431,622
Short-term Investments        
Short-term investments (due within 1 year)        
Available-for-sale Securities 55,901,729 106,979,224    
Available-for-sale Securities, Amortized Cost Basis 55,907,026 107,020,420    
Short-term Investments | U.S. government treasury obligations        
Short-term investments (due within 1 year)        
Available-for-sale Securities 36,953,105 86,053,755    
Available-for-sale Securities, Amortized Cost Basis 36,957,883 86,078,622    
Short-term Investments | Corporate obligations        
Short-term investments (due within 1 year)        
Available-for-sale Securities 18,948,624 20,925,469    
Available-for-sale Securities, Amortized Cost Basis 18,949,143 20,941,799    
Cash Equivalents [Member]        
Cash equivalents        
Cash and cash equivalents at cost 47,177,769 31,228,192    
Cash and cash equivalents at carrying value 47,178,505 31,228,192    
FDIC insured deposits and money market funds        
Cash equivalents        
Cash equivalents at cost   24,978,489    
Cash Equivalents, at Carrying Value   24,978,489    
Repurchase Agreements [Member] | FDIC insured deposits and money market funds        
Cash equivalents        
Cash equivalents at cost 15,000,000      
Cash Equivalents, at Carrying Value 15,000,000      
Money Market Funds [Member]        
Cash equivalents        
Cash equivalents at cost 14,573,254      
Cash Equivalents, at Carrying Value 14,573,254      
U.S. government treasury obligations        
Cash equivalents        
Cash equivalents at cost 9,495,984      
Cash Equivalents, at Carrying Value 9,496,720      
Corporate obligations        
Cash equivalents        
Cash equivalents at cost 1,998,361      
Cash Equivalents, at Carrying Value 1,998,361      
Level 1        
Cash        
Cash 6,110,170 6,249,703    
Level 1 | Short-term Investments        
Short-term investments (due within 1 year)        
Available-for-sale Securities 36,953,105 86,053,755    
Level 1 | Short-term Investments | U.S. government treasury obligations        
Short-term investments (due within 1 year)        
Available-for-sale Securities 36,953,105 86,053,755    
Level 1 | Cash Equivalents [Member]        
Cash equivalents        
Cash and cash equivalents at carrying value 30,180,144 31,228,192    
Level 1 | FDIC insured deposits and money market funds        
Cash equivalents        
Cash Equivalents, at Carrying Value   24,978,489    
Level 1 | Money Market Funds [Member]        
Cash equivalents        
Cash Equivalents, at Carrying Value 14,573,254      
Level 1 | U.S. government treasury obligations        
Cash equivalents        
Cash Equivalents, at Carrying Value 9,496,720      
Level 2 | Short-term Investments        
Short-term investments (due within 1 year)        
Available-for-sale Securities 18,948,624 20,925,469    
Level 2 | Short-term Investments | Corporate obligations        
Short-term investments (due within 1 year)        
Available-for-sale Securities 18,948,624 $ 20,925,469    
Level 2 | Cash Equivalents [Member]        
Cash equivalents        
Cash and cash equivalents at carrying value 16,998,361      
Level 2 | Repurchase Agreements [Member] | FDIC insured deposits and money market funds        
Cash equivalents        
Cash Equivalents, at Carrying Value 15,000,000      
Level 2 | Corporate obligations        
Cash equivalents        
Cash Equivalents, at Carrying Value $ 1,998,361