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Summary of significant accounting policies - Debt Issuance Costs (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Summary of significant accounting policies          
Debt issuance costs $ 0   $ 0   $ 1,300,000
Amortization of the debt issuance costs $ 100,000 $ 100,000 $ 400,000 $ 700,000