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Summary of Significant Accounting Policies - Cash and cash equivalents & Deferred offering costs (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Sep. 30, 2020
Dec. 31, 2019
Summary of Significant Accounting Policies        
Cash and cash equivalents $ 416,850 $ 271,382 $ 29,937  
Restricted cash, current 1,108 1,909 1,812  
Total cash and cash equivalents, and restricted cash $ 417,958 $ 273,291 $ 31,749 $ 29,762