Commitments and contingencies - UMass and MassBiologics Collaboration - (Details) - USD ($)	1 Months Ended		6 Months Ended	12 Months Ended
	Nov. 30, 2014	Jan. 31, 2014	Dec. 31, 2013	Dec. 31, 2014
Common Stock
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock (in shares)			2,352
University Of Massachusetts (UMass) Collaboration Agreement | Common Stock
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock (in shares)		23,529
Issuance of common stock		$ 12,000
Umass and MassBiologics Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount funded to collaborative partner for research and devlopment services to be performed	$ 2,861,000
Term of funding to collaborative partner for research and devlopment services to be performed	16 months
Cumulative funding provided to collaborative partner for research and devlopment services				$ 376,000
Cumulative funding in excess of costs incurred for research and devlopment services performed by collaborative partner				$ 376,000