Net income per share (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net income	$ 3,695,099	$ 2,225,413	$ 7,082,168	$ 5,268,441
Amounts allocated to participating preferred stockholders under the two-class method	(319,921)	(196,868)	(613,173)	(466,065)
Net income applicable to common stockholders (basic and dilutive)	$ 3,375,178	$ 2,028,545	$ 6,468,995	$ 4,802,376
Denominator:
Weighted-average common stock outstanding (basic) (in shares)	49,391,614	49,179,596	49,184,276	49,848,634
Common equivalent shares from options to purchase common stock (in shares)	1,870,155	2,074,627	2,112,743	2,153,615
Weighted-average common stock outstanding (diluted) (in shares)	51,261,769	51,254,223	51,297,019	52,002,249
Basic net income per share applicable to common stockholders ( in dollars per share)	$ 0.07	$ 0.04	$ 0.13	$ 0.10
Diluted net income per share applicable to common stockholders (in dollars per share)	$ 0.07	$ 0.04	$ 0.13	$ 0.09
Total anti-dilutive securities (in shares)	4,729,737	4,688,237	4,747,396	6,098,646
Contingently issued restricted shares
Denominator:
Total anti-dilutive securities (in shares)	396,500	396,500	396,500	396,500
Stock options
Denominator:
Total anti-dilutive securities (in shares)	41,500		28,921	581,665
Restricted stock
Denominator:
Total anti-dilutive securities (in shares)			30,238	23,574
Convertible preferred stock
Denominator:
Total anti-dilutive securities (in shares)	4,688,237	4,688,237	4,688,237	5,493,407