Summary of Significant Accounting Policies (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	$ 585,575	$ 84,441	$ 97,187
Restricted cash	4,556	3,777	3,000
Total cash, cash equivalents and restricted cash in the Condensed Statements of Cash Flows	$ 590,131	$ 88,218	$ 100,187	$ 53,436