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Summary of significant accounting policies - Deferred offering costs (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2020
Dec. 31, 2019
Summary of significant accounting policies      
Deferred offering costs $ 5,800 $ 5,800 $ 2,300
Offering costs $ 700 $ 2,203