Summary of Significant Accounting Policies (Components of Cash, Cash Equivalents and Restricted Cash) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Cash and cash equivalents	$ 352,842	$ 371,341	$ 584,581
Restricted cash and cash equivalents included in other current assets	10,374	4,423	2,697
Restricted cash and cash equivalents included in other assets	2,461	3,712	4,451
Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows	$ 365,677	$ 379,476	$ 591,729	$ 310,937