Fair value measurements (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Marketable securities:
Marketable securities	$ 38,658,933	$ 26,965,297
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	11,681,074	59,985,670
Marketable securities:
Total assets	50,340,007	86,950,967
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	11,681,074	58,575,348
Marketable securities:
Total assets	11,681,074	58,575,348
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		1,410,322
Marketable securities:
Total assets	38,658,933	28,375,619
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
Marketable securities:
Total assets	0	0
Fair Value, Measurements, Recurring | Corporate debt securities
Marketable securities:
Marketable securities	1,518,240	26,965,297
Fair Value, Measurements, Recurring | Corporate debt securities | Level 1
Marketable securities:
Marketable securities	0	0
Fair Value, Measurements, Recurring | Corporate debt securities | Level 2
Marketable securities:
Marketable securities	1,518,240	26,965,297
Fair Value, Measurements, Recurring | Corporate debt securities | Level 3
Marketable securities:
Marketable securities	0	$ 0
Fair Value, Measurements, Recurring | Commercial paper
Marketable securities:
Marketable securities	6,081,202
Fair Value, Measurements, Recurring | Commercial paper | Level 1
Marketable securities:
Marketable securities	0
Fair Value, Measurements, Recurring | Commercial paper | Level 2
Marketable securities:
Marketable securities	6,081,202
Fair Value, Measurements, Recurring | Commercial paper | Level 3
Marketable securities:
Marketable securities	0
Fair Value, Measurements, Recurring | U.S. government agency securities
Marketable securities:
Marketable securities	31,059,491
Fair Value, Measurements, Recurring | U.S. government agency securities | Level 1
Marketable securities:
Marketable securities	0
Fair Value, Measurements, Recurring | U.S. government agency securities | Level 2
Marketable securities:
Marketable securities	31,059,491
Fair Value, Measurements, Recurring | U.S. government agency securities | Level 3
Marketable securities:
Marketable securities	$ 0