Commitments and contingencies - UMass and MassBiologics Collaboration - (Details) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Nov. 30, 2014	Jan. 31, 2014	Dec. 31, 2015
Common Stock
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Stock Issued During Period Shares New Issues			5,750,000
University Of Massachusetts (UMass) Collaboration Agreement | Common Stock
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Stock Issued During Period Shares New Issues		23,529
Issuance of common stock		$ 12
Umass and MassBiologics Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount funded to collaborative partner for research and development services to be performed.	$ 2,900
Term of funding to collaborative partner for research and devlopment services to be performed	16 months