Summary of significant accounting policies - Sources of Liquidity and Funding Requirements (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	May 31, 2019	Mar. 31, 2020	Dec. 31, 2019
Summary of significant accounting policies
Net proceeds from convertible preferred shares to accredited investors		$ 138,800
Net proceeds from initial public offering	$ 85,400
Cash and cash equivalents		101,816	$ 119,818
Accumulated deficit		$ (129,842)	$ (113,499)