Shareholders' equity - Extinguishment of derivative obligations (Details) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Aug. 07, 2015	Jun. 12, 2015	Dec. 31, 2015	Apr. 15, 2015	Feb. 05, 2014
Price of shares issued (in dollars per share)				$ 29.50	$ 17.00
BMS
Balance			$ 71,931
BMS \| Common shares
Balance			$ 132
Balance (in shares)			2,388,108
BMS \| Additional paid-in capital
Balance			$ 71,799
Collaboration arrangement \| BMS
Issuance of shares to collaboration partner	$ 39,300	$ 32,600
Issuance of shares to collaboration partner (in shares)	1,300,000	1,100,000
Extinguishment of derivative upon issuance of shares to collaboration partner	$ 1,400	$ 5,000
Price of shares issued (in dollars per share)	$ 29.67	$ 33.84
BMS on June 12, 2015
Issuance of shares to collaboration partner			71,931
BMS on June 12, 2015 \| Common shares
Issuance of shares to collaboration partner			$ 132
Issuance of shares to collaboration partner (in shares)			2,388,108
BMS on June 12, 2015 \| Additional paid-in capital
Issuance of shares to collaboration partner			$ 71,799
BMS on June 12, 2015 \| BMS
Issuance of shares to collaboration partner			37,640
Extinguishment of derivative upon issuance of shares to collaboration partner			(4,972)
BMS on June 12, 2015 \| BMS \| Common shares
Issuance of shares to collaboration partner			$ 61
Issuance of shares to collaboration partner (in shares)			1,112,319
BMS on June 12, 2015 \| BMS \| Additional paid-in capital
Issuance of shares to collaboration partner			$ 37,579
Extinguishment of derivative upon issuance of shares to collaboration partner			(4,972)
BMS on August 7, 2015 \| BMS
Issuance of shares to collaboration partner			37,853
Extinguishment of derivative upon issuance of shares to collaboration partner			1,410
BMS on August 7, 2015 \| BMS \| Common shares
Issuance of shares to collaboration partner			$ 71
Issuance of shares to collaboration partner (in shares)			1,275,789
BMS on August 7, 2015 \| BMS \| Additional paid-in capital
Issuance of shares to collaboration partner			$ 37,782
Extinguishment of derivative upon issuance of shares to collaboration partner			$ 1,410