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Investments (Details) - USD ($)
Sep. 30, 2018
Dec. 31, 2017
Cash    
Cash $ 7,280,004 $ 2,544,972
Cash equivalents    
Cash and cash equivalents at carrying value 67,268,340 18,559,130
Short-term investments (due within 1 year)    
Available-for-sale securities 276,942,151 78,912,297
Short-term Investments    
Short-term investments (due within 1 year)    
Available-for-sale securities, amortized cost 276,964,699 78,976,730
Available-for-sale securities 276,942,151 78,912,297
Short-term Investments | U.S. government treasury obligations    
Short-term investments (due within 1 year)    
Available-for-sale securities, amortized cost 187,315,613 63,034,548
Available-for-sale securities 187,297,850 62,970,115
Short-term Investments | Corporate obligations    
Short-term investments (due within 1 year)    
Available-for-sale securities, amortized cost 89,649,086 15,942,182
Available-for-sale securities 89,644,301 15,942,182
Cash Equivalents [Member]    
Cash equivalents    
Cash and cash equivalents at cost 67,268,415 18,559,130
Cash and cash equivalents at carrying value 67,268,340 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 35,004,236 6,014,158
Cash Equivalents, at Carrying Value 35,004,236 6,014,158
U.S. government agency obligations    
Cash equivalents    
Cash equivalents at cost 14,984,175  
Cash Equivalents, at Carrying Value 14,984,100  
Level 1    
Cash    
Cash 7,280,004 2,544,972
Level 1 | Short-term Investments    
Short-term investments (due within 1 year)    
Available-for-sale securities 187,297,850 62,970,115
Level 1 | Short-term Investments | U.S. government treasury obligations    
Short-term investments (due within 1 year)    
Available-for-sale securities 187,297,850 62,970,115
Level 1 | Cash Equivalents [Member]    
Cash equivalents    
Cash and cash equivalents at carrying value 42,284,240 8,559,130
Level 1 | Money Market Funds    
Cash equivalents    
Cash Equivalents, at Carrying Value 35,004,236 6,014,158
Level 2 | Short-term Investments    
Short-term investments (due within 1 year)    
Available-for-sale securities 89,644,301 15,942,182
Level 2 | Short-term Investments | Corporate obligations    
Short-term investments (due within 1 year)    
Available-for-sale securities 89,644,301 15,942,182
Level 2 | Cash Equivalents [Member]    
Cash equivalents    
Cash and cash equivalents at carrying value 24,984,100 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 | U.S. government agency obligations    
Cash equivalents    
Cash Equivalents, at Carrying Value $ 14,984,100