Net income per share (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator:
Net income	$ 1,969,529	$ 1,282,723	$ 3,387,069	$ 3,043,028
Amounts allocated to participating preferred stockholders under the two-class method	(170,881)	(110,402)	(293,870)	(261,909)
Net income applicable to common stockholders (basic and dilutive)	$ 1,798,648	$ 1,172,321	$ 3,093,199	$ 2,781,119
Denominator:
Weighted-average common stock outstanding (basic) (in shares)	49,238,405	49,602,206	49,078,889	50,186,828
Common equivalent shares from options to purchase common stock (in shares)	2,033,875	2,385,489	2,243,407	2,214,962
Weighted-average common stock outstanding (diluted) (in shares)	51,272,280	51,987,695	51,322,296	52,401,790
Basic net income per share applicable to common stockholders ( in dollars per share)	$ 0.04	$ 0.02	$ 0.06	$ 0.06
Diluted net income per share applicable to common stockholders (in dollars per share)	$ 0.04	$ 0.02	$ 0.06	$ 0.05
Total anti-dilutive securities (in shares)	4,738,925	4,960,758	4,719,661	6,637,513
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)	37,390	11,071	22,528	592,170
Restricted stock
Denominator:
Total anti-dilutive securities (in shares)	13,298		8,896	30,004
Convertible preferred stock
Denominator:
Total anti-dilutive securities (in shares)	4,688,237	4,949,687	4,688,237	6,015,339