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Summary of significant accounting policies (Details)
12 Months Ended
Dec. 31, 2014
item
Segment and geographic information  
Number of operating segments 1us-gaap_NumberOfOperatingSegments
Computer equipment and software  
Fixed assets  
Estimated useful life 3 years
Furniture, fixtures, and lab equipment | Minimum  
Fixed assets  
Estimated useful life 3 years
Furniture, fixtures, and lab equipment | Maximum  
Fixed assets  
Estimated useful life 7 years