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Summary of significant accounting policies - Income Taxes (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Summary of significant accounting policies      
Income tax benefit recognition threshold 50.00%    
Unrecognized tax benefit or accrued interest and penalties $ 0 $ 0 $ 0