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Summary of significant accounting policies - Advertising Costs (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Maximum        
Class of Stock [Line Items]        
Advertising costs $ 0.1 $ 0.1 $ 0.1 $ 0.1