Net income per share (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Numerator:
Net income	$ 1,760,305	$ 1,711,626
Amounts allocated to participating preferred stockholders under the two-class method	(210,498)	(193,285)
Net income applicable to common stockholders (basic and dilutive)	$ 1,549,807	$ 1,518,341
Denominator:
Weighted-average common stock outstanding (basic) (in shares)	50,771,451	53,800,606
Common equivalent shares from options and warrants to purchase common stock (in shares)	1,291,048	647,776
Weighted average common stock outstanding (diluted) (in shares)	52,062,499	54,448,382
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)	8,784,615	12,985,556
Contingently issued restricted shares
Denominator:
Total anti-dilutive securities (in shares)	996,500	1,035,180