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Summary of significant accounting policies - Restricted cash (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Jun. 30, 2021
Dec. 31, 2020
Summary of significant accounting policies      
Separate cash balance for the landlord $ 108   $ 108
Separate cash balance to collateralize corporate credit cards with a bank     $ 40
Aggregate market value of stock   $ 700,000