Stock-based compensation - Summary of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Total stock-based compensation expense	$ 1,022	$ 788	$ 1,705	$ 1,866
Research and Development Expense [Member]
Total stock-based compensation expense	346		629	80
General and Administrative Expense [Member]
Total stock-based compensation expense	$ 676	$ 788	$ 1,076	$ 1,786