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Summary of Significant Accounting Policies (Cash and Cash Equivalents) (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Accounting Policies [Abstract]    
Cash and cash equivalents $ 780,714 $ 814,354
Overnight money market funds 721,700 752,400
Restricted cash $ 5,150 $ 7,165