Summary of significant accounting policies - Net income per share (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net income	$ 25,072,002	$ 10,362,448	$ 3,160,385
Amounts allocated to participating preferred stockholders under the two-class method	(2,202,318)	(1,218,970)	(357,248)
Net income applicable to common stockholders (basic and dilutive)	$ 22,869,684	$ 9,143,478	$ 2,803,137
Denominator:
Weighted-average common stock outstanding (basic) (in shares)	49,472,512	52,545,934	53,494,952
Common equivalent shares from options and warrants to purchase common stock (in shares)	2,088,286	725,314	1,138,229
Weighted-average common stock outstanding (diluted) (in shares)	51,560,798	53,271,248	54,633,181
Basic net income per share applicable to common stockholders ( in dollars per share)	$ 0.46	$ 0.17	$ 0.05
Diluted net income per share applicable to common stockholders (in dollars per share)	$ 0.44	$ 0.17	$ 0.05
Total anti-dilutive securities (in shares)	5,326,693	11,857,746	12,117,389
Contingently issued restricted shares
Denominator:
Total anti-dilutive securities (in shares)	400,000	1,000,000	1,300,000
Stock options
Denominator:
Total anti-dilutive securities (in shares)	3,135	4,897,096	5,107,474
Restricted stock
Denominator:
Total anti-dilutive securities (in shares)	32,544	109,506	90,467
Warrants
Denominator:
Total anti-dilutive securities (in shares)			68,304
Convertible preferred stock
Denominator:
Total anti-dilutive securities (in shares)	5,291,014	6,851,144	6,851,144