Summary of Significant Accounting Policies (Reconciliation of Cash, Cash Equivalents, and Restricted Cash) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Cash and cash equivalents	$ 490,390	$ 352,842
Restricted cash and cash equivalents included in other current assets	11,364	10,374
Restricted cash and cash equivalents included in other assets	3,343	2,461
Total cash, cash equivalents, and restricted cash shown in the condensed consolidated statements of cash flows	$ 505,097	$ 365,677	$ 549,134	$ 379,476