Summary of significant accounting policies - Net income per share (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net income	$ 10,362,448	$ 3,160,385	$ 22,006
Amounts allocated to participating preferred stockholders under the two-class method	(1,218,970)	(357,248)	(2,507)
Net income applicable to common stockholders (basic and dilutive)	$ 9,143,478	$ 2,803,137	$ 19,499
Denominator:
Weighted-average common stock outstanding (basic) (in shares)	52,545,934	53,494,952	52,943,203
Common equivalent shares from options and warrants to purchase common stock (in shares)	725,314	1,138,229	1,522,082
Weighted average common stock outstanding (diluted) (in shares)	53,271,248	54,633,181	54,465,285
Basic net income per share applicable to common stockholders ( in dollars per share)	$ 0.17	$ 0.05	$ 0.00
Diluted net income per share applicable to common stockholders (in dollars per share)	$ 0.17	$ 0.05	$ 0.00
Total anti-dilutive securities (in shares)	11,857,746	12,117,389	9,395,244
Contingently issued restricted shares
Denominator:
Total anti-dilutive securities (in shares)	1,000,000	1,300,000	1,600,000