Stock incentive plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		18 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance	4,405,755		4,405,755		4,405,755
Stock-based compensation expense	$ 740	$ 221	$ 1,264	$ 412
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	243	76	375	143
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	497	145	$ 889	$ 269
Restricted stock awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 100	$ 100			$ 200
2015 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance	505,046		505,046		505,046