Net income per share (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Numerator:
Net income	$ 1,417,540	$ 1,760,305
Amounts allocated to participating preferred stockholders under the two-class method	(123,537)	(210,498)
Net income applicable to common stockholders (basic and dilutive)	$ 1,294,003	$ 1,549,807
Denominator:
Weighted-average common stock outstanding (basic) (in shares)	48,917,601	50,771,451
Common equivalent shares from options to purchase common stock (in shares)	2,436,206	1,291,048
Weighted-average common stock outstanding (diluted) (in shares)	51,353,807	52,062,499
Basic net income per share applicable to common stockholders ( in dollars per share)	$ 0.03	$ 0.03
Diluted net income per share applicable to common stockholders (in dollars per share)	$ 0.03	$ 0.03
Total anti-dilutive securities (in shares)	4,695,737	8,784,615
Contingently issued restricted shares
Denominator:
Total anti-dilutive securities (in shares)	396,500	996,500
Stock options
Denominator:
Total anti-dilutive securities (in shares)	7,500	1,933,471
Convertible preferred stock
Denominator:
Total anti-dilutive securities (in shares)	4,688,237	6,851,144