Net income per share (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Numerator:
Net income	$ 2,225,413	$ 105,971	$ 5,268,441	$ 3,692,925
Amounts allocated to participating preferred stockholders under the two-class method	(196,868)	(12,284)	(466,065)	(428,090)
Net income applicable to common stockholders (basic and dilutive)	$ 2,028,545	$ 93,687	$ 4,802,376	$ 3,264,835
Denominator:
Weighted-average common stock outstanding (basic) (in shares)	49,179,596	51,360,758	49,848,634	52,985,106
Common equivalent shares from options and warrants to purchase common stock (in shares)	2,074,627	1,023,633	2,153,615	626,862
Weighted-average common stock outstanding (diluted) (in shares)	51,254,223	52,384,391	52,002,249	53,611,968
Basic net income per share applicable to common stockholders ( in dollars per share)	$ 0.04	$ 0.00	$ 0.10	$ 0.06
Diluted net income per share applicable to common stockholders (in dollars per share)	$ 0.04	$ 0.00	$ 0.09	$ 0.06
Total anti-dilutive securities (in shares)	4,688,237	10,440,006	6,098,646	12,045,310
Contingently issued restricted shares
Denominator:
Total anti-dilutive securities (in shares)	396,500	1,446,500	396,500	1,446,500