Nature of the business and basis of presentation - Initial public offering (Details) $ / shares in Units, $ in Millions			1 Months Ended	3 Months Ended	6 Months Ended
	May 28, 2019 shares	Mar. 28, 2019 $ / shares shares	Jun. 30, 2019 shares	Jun. 30, 2019 USD ($) shares	Jun. 30, 2020 USD ($)
Share capital reorganization and share split
Offering expenses | $				$ 8.4
Ordinary Shares
Share capital reorganization and share split
Shares issued (in shares)				4,637,666
Conversion of convertible preferred shares to ordinary shares (in shares)				11,647,529
IPO
Share capital reorganization and share split
Shares issued (in shares)		4,333,333
Share issue price | $ / shares		$ 14.00
Proceeds from IPO, net | $					$ 56.4
Underwriting discounts and commissions | $					4.5
Offering expenses | $					$ 4.0
IPO | Ordinary Shares
Share capital reorganization and share split
Conversion of convertible preferred shares to ordinary shares (in shares)	11,647,529
Conversion ratio	0.6997
Over allotment option
Share capital reorganization and share split
Shares issued (in shares)			304,333