Significant accounting policies (Details 4) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
item
Property and equipment  
Impairment losses on long-lived assets $ 0
Concentrations of credit risk and off-balance sheet risk  
Number of financial institutions in which cash, cash equivalents and investments were deposited 2
Significant off-balance sheet concentrations of credit risk 0.00%
Laboratory equipment  
Property and equipment  
Estimated useful lives 5 years
Furniture  
Property and equipment  
Estimated useful lives 5 years
Computer equipment  
Property and equipment  
Estimated useful lives 3 years