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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
Summary of Significant Accounting Policies        
Advertising costs $ 2.7 $ 0.0 $ 8.8 $ 0.0