Stock incentive plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance					3,878,940
Stock-based compensation expense	$ 720	$ 242	$ 1,984	$ 653
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	242	91	616	233
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	478	151	1,368	420
Restricted stock awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 100	$ 100	$ 300	$ 300
2015 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance	505,046		505,046