Summary of significant accounting policies (Details 6)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Anti-dilutive securities excluded from computation of diluted earnings per share
Total anti-dilutive securities (in shares)	12,117,389	9,395,244	17,435,682
Contingently issued restricted shares
Anti-dilutive securities excluded from computation of diluted earnings per share
Total anti-dilutive securities (in shares)	1,300,000	1,600,000	900,000
Stock options
Anti-dilutive securities excluded from computation of diluted earnings per share
Total anti-dilutive securities (in shares)	5,107,474	2,458,471	8,528,816
Restricted stock awards
Anti-dilutive securities excluded from computation of diluted earnings per share
Total anti-dilutive securities (in shares)	90,467
Warrants
Anti-dilutive securities excluded from computation of diluted earnings per share
Total anti-dilutive securities (in shares)	68,304	85,629	2,055,722
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