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Summary of Significant Accounting Policies - Cash Equivalents and Restricted Cash (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Sep. 30, 2020
Dec. 31, 2019
Accounting Policies [Abstract]        
Cash and cash equivalents $ 448,608 $ 15,856    
Restricted cash 0 4,370    
Restricted cash – non–current 400,000 0    
Total cash, cash equivalents and restricted cash $ 848,608 $ 20,226 $ 84,265 $ 20,019