Significant accounting policies - Fair value of financial instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair value of financial instruments
Cash equivalents	$ 109,155	$ 42,608
Note receivable	150	150
Total	109,305	42,758
Level 1
Fair value of financial instruments
Cash equivalents	109,155	42,608
Total	109,155	42,608
Level 3
Fair value of financial instruments
Note receivable	150	150
Total	$ 150	$ 150