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Summary of Significant Accounting Policies - Cash and Cash Equivalents and Restricted Cash (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Sep. 30, 2020
Dec. 31, 2019
Aggregate amounts of cash and cash equivalents and the restricted cash        
Cash and cash equivalents $ 120,542 $ 117,358 $ 83,625  
Restricted cash - current   10 10  
Restricted cash - noncurrent 225 450 450  
Total cash reported on condensed consolidated statements of cash flows 120,767 117,818 $ 84,085 $ 33,466
Letter of credit        
Aggregate amounts of cash and cash equivalents and the restricted cash        
Restricted cash - noncurrent $ 200 $ 500