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Fair value measurements (Details) - USD ($)
Sep. 30, 2016
Dec. 31, 2015
Marketable securities:    
Fair Value $ 39,504,263 $ 26,965,297
Corporate debt securities    
Marketable securities:    
Fair Value 11,418,230  
U.S. government agency securities    
Marketable securities:    
Fair Value 28,086,033  
Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 20,697,042 59,985,670
Marketable securities:    
Total assets 60,201,305 86,950,967
Fair Value, Measurements, Recurring | Corporate debt securities    
Marketable securities:    
Fair Value 11,418,230 26,965,297
Fair Value, Measurements, Recurring | U.S. government agency securities    
Marketable securities:    
Fair Value 28,086,033  
Fair Value, Measurements, Recurring | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 20,697,042 58,575,348
Marketable securities:    
Total assets 20,697,042 58,575,348
Fair Value, Measurements, Recurring | Level 1 | Corporate debt securities    
Marketable securities:    
Fair Value 0 0
Fair Value, Measurements, Recurring | Level 1 | U.S. government agency securities    
Marketable securities:    
Fair Value 0  
Fair Value, Measurements, Recurring | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0 1,410,322
Marketable securities:    
Total assets 39,504,263 28,375,619
Fair Value, Measurements, Recurring | Level 2 | Corporate debt securities    
Marketable securities:    
Fair Value 11,418,230 26,965,297
Fair Value, Measurements, Recurring | Level 2 | U.S. government agency securities    
Marketable securities:    
Fair Value 28,086,033  
Fair Value, Measurements, Recurring | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0 0
Marketable securities:    
Total assets 0 0
Fair Value, Measurements, Recurring | Level 3 | Corporate debt securities    
Marketable securities:    
Fair Value 0 $ 0
Fair Value, Measurements, Recurring | Level 3 | U.S. government agency securities    
Marketable securities:    
Fair Value $ 0