Summary of significant accounting policies - Anti-dilutive securities (Details) - shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Anti-dilutive securities excluded from computation of diluted earnings per share
Total anti-dilutive securities (in shares)	11,857,746	12,117,389	9,395,244
Stock options
Anti-dilutive securities excluded from computation of diluted earnings per share
Total anti-dilutive securities (in shares)	4,897,096	5,107,474	2,458,471
Restricted stock
Anti-dilutive securities excluded from computation of diluted earnings per share
Total anti-dilutive securities (in shares)	109,506	90,467
Warrants
Anti-dilutive securities excluded from computation of diluted earnings per share
Total anti-dilutive securities (in shares)		68,304	85,629
Convertible preferred stock
Anti-dilutive securities excluded from computation of diluted earnings per share
Total anti-dilutive securities (in shares)	6,851,144	6,851,144	6,851,144