Property and equipment (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Property and equipment      
Property and equipment, gross $ 3,686,000 $ 1,144,000  
Less: accumulated depreciation (861,000) (513,000)  
Property and equipment, net 2,825,000 631,000  
Depreciation expenses 427,000 236,000 207,000
Laboratory equipment      
Property and equipment      
Property and equipment, gross 1,040,000 1,035,000  
Computer equipment      
Property and equipment      
Property and equipment, gross 225,000 27,000  
Furniture      
Property and equipment      
Property and equipment, gross 325,000 44,000  
Leasehold Improvements      
Property and equipment      
Property and equipment, gross $ 2,096,000 $ 38,000