Nature of the business and basis of presentation - Initial public offering (Details) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended	12 Months Ended
	May 28, 2019 $ / shares shares	Jun. 30, 2019 shares	Jun. 30, 2019 shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Share capital reorganization and share split
Offering expenses | $				$ 8,500	$ 327	$ 587
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	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 | $				$ 56,400
Underwriting discounts and commissions | $				4,500
Offering expenses | $				$ 4,000
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