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Summary of Significant Accounting Policies (Cash and Cash Equivalents) (Details) - USD ($)
$ in Thousands
Jun. 30, 2021
Dec. 31, 2020
Accounting Policies [Abstract]    
Cash and cash equivalents $ 906,409 $ 814,354
Overnight money market funds 878,400 752,400
Restricted cash $ 8,839 $ 7,165