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Investments (Details) - USD ($)
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Cash        
Cash $ 6,249,703 $ 5,154,191    
Cash equivalents        
FDIC insured deposits and money market funds 24,978,489 10,277,431    
Cash and cash equivalents 31,228,192 15,431,622 $ 45,533,443 $ 52,846,940
Short-term Investments        
Short-term investments (due within 1 year)        
Available-for-sale Securities 106,979,224 158,168,832    
Available-for-sale Securities, Amortized Cost Basis 107,020,420 158,248,231    
Short-term Investments | U.S. government treasury obligations        
Short-term investments (due within 1 year)        
Available-for-sale Securities 86,053,755 73,560,085    
Available-for-sale Securities, Amortized Cost Basis 86,078,622 73,593,081    
Short-term Investments | U.S. government agency obligations        
Short-term investments (due within 1 year)        
Available-for-sale Securities   55,670,043    
Available-for-sale Securities, Amortized Cost Basis   55,702,099    
Short-term Investments | Corporate obligations        
Short-term investments (due within 1 year)        
Available-for-sale Securities 20,925,469 28,938,704    
Available-for-sale Securities, Amortized Cost Basis 20,941,799 28,953,051    
Level 1        
Cash        
Cash 6,249,703 5,154,191    
Cash equivalents        
FDIC insured deposits and money market funds 24,978,489 10,277,431    
Cash and cash equivalents 31,228,192 15,431,622    
Level 1 | Short-term Investments        
Short-term investments (due within 1 year)        
Available-for-sale Securities 86,053,755 129,230,128    
Level 1 | Short-term Investments | U.S. government treasury obligations        
Short-term investments (due within 1 year)        
Available-for-sale Securities 86,053,755 73,560,085    
Level 1 | Short-term Investments | U.S. government agency obligations        
Short-term investments (due within 1 year)        
Available-for-sale Securities   55,670,043    
Level 1 | Short-term Investments | Corporate obligations        
Short-term investments (due within 1 year)        
Available-for-sale Securities 0      
Level 2        
Cash        
Cash 0      
Cash equivalents        
FDIC insured deposits and money market funds 0      
Cash and cash equivalents 0      
Level 2 | Short-term Investments        
Short-term investments (due within 1 year)        
Available-for-sale Securities 20,925,469 28,938,704    
Level 2 | Short-term Investments | U.S. government treasury obligations        
Short-term investments (due within 1 year)        
Available-for-sale Securities 0      
Level 2 | Short-term Investments | Corporate obligations        
Short-term investments (due within 1 year)        
Available-for-sale Securities $ 20,925,469 $ 28,938,704