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Investments (Details) - USD ($)
Jun. 30, 2018
Dec. 31, 2017
Cash    
Cash $ 5,466,279 $ 2,544,972
Cash equivalents    
Cash and cash equivalents at carrying value 25,044,956 18,559,130
Short-term Investments    
Short-term investments (due within 1 year)    
Available-for-sale securities, amortized cost 141,782,048 78,976,730
Available-for-sale securities 141,747,724 78,912,297
Short-term Investments | U.S. government treasury obligations    
Short-term investments (due within 1 year)    
Available-for-sale securities, amortized cost 102,051,027 63,034,548
Available-for-sale securities 102,023,195 62,970,115
Short-term Investments | Corporate obligations    
Short-term investments (due within 1 year)    
Available-for-sale securities, amortized cost 39,731,021 15,942,182
Available-for-sale securities 39,724,529 15,942,182
Cash Equivalents [Member]    
Cash equivalents    
Cash and cash equivalents at cost 25,044,956 18,559,130
Cash and cash equivalents at carrying value 25,044,956 18,559,130
FDIC insured deposits and money market funds    
Cash equivalents    
Cash equivalents at cost   10,000,000
Cash Equivalents, at Carrying Value   10,000,000
Repurchase Agreements    
Cash equivalents    
Cash equivalents at cost 10,000,000  
Cash Equivalents, at Carrying Value 10,000,000  
Money Market Funds    
Cash equivalents    
Cash equivalents at cost 6,085,782 6,014,158
Cash Equivalents, at Carrying Value 6,085,782 6,014,158
Corporate obligations    
Cash equivalents    
Cash equivalents at cost 3,492,895  
Cash Equivalents, at Carrying Value 3,492,895  
Level 1    
Cash    
Cash 5,466,279 2,544,972
Level 1 | Short-term Investments    
Short-term investments (due within 1 year)    
Available-for-sale securities 102,023,195 62,970,115
Level 1 | Short-term Investments | U.S. government treasury obligations    
Short-term investments (due within 1 year)    
Available-for-sale securities 102,023,195 62,970,115
Level 1 | Cash Equivalents [Member]    
Cash equivalents    
Cash and cash equivalents at carrying value 11,552,061 8,559,130
Level 1 | Money Market Funds    
Cash equivalents    
Cash Equivalents, at Carrying Value 6,085,782 6,014,158
Level 2 | Short-term Investments    
Short-term investments (due within 1 year)    
Available-for-sale securities 39,724,529 15,942,182
Level 2 | Short-term Investments | Corporate obligations    
Short-term investments (due within 1 year)    
Available-for-sale securities 39,724,529 15,942,182
Level 2 | Cash Equivalents [Member]    
Cash equivalents    
Cash and cash equivalents at carrying value 13,492,895 10,000,000
Level 2 | FDIC insured deposits and money market funds    
Cash equivalents    
Cash Equivalents, at Carrying Value   $ 10,000,000
Level 2 | Repurchase Agreements    
Cash equivalents    
Cash Equivalents, at Carrying Value 10,000,000  
Level 2 | Corporate obligations    
Cash equivalents    
Cash Equivalents, at Carrying Value $ 3,492,895