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Net loss per share (Details 2) - shares
6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Net loss per share    
Total shares excluded from calculation 5,016,987 3,912,283
Stock options    
Net loss per share    
Total shares excluded from calculation 4,663,852 3,182,963
Unvested restricted stock    
Net loss per share    
Total shares excluded from calculation 353,135 729,320