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Organization and basis of presentation (Details)
$ in Thousands
Jun. 30, 2016
USD ($)
treatment
Dec. 31, 2015
USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]    
Accumulated deficit | $ $ (208,243) $ (167,917)
Number of treatments | treatment 1  
Potential treatments in development | treatment 2  
Cash, cash equivalents and short-term investments | $ $ 147,700