Summary of significant accounting policies (Details 5) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator:
Net income (loss)	$ 3,160,385	$ 22,006	$ (4,452,305)
Amounts allocated to participating preferred stockholders under the two-class method	(357,248)	(2,507)
Net income (loss) applicable to common stockholders - basic	2,803,137	19,499	(4,452,305)
Net income (loss) applicable to common stockholders - dilutive	$ 2,803,137	$ 19,499	$ (4,452,305)
Denominator:
Weighted average common stock outstanding (basic) (in shares)	53,494,952	52,943,203	52,187,144
Common equivalent shares from options and warrants to purchase common stock (in shares)	1,138,229	1,522,082
Weighted average common stock outstanding (diluted) (in shares)	54,633,181	54,465,285	52,187,144
Basic net income (loss) per share applicable to common stockholders ( in dollars per share)	$ 0.05	$ 0.00	$ (0.09)
Diluted net income (loss) per share applicable to common stockholders (in dollars per share)	$ 0.05	$ 0.00	$ (0.09)