Summary of Significant Accounting Policies (Reconciliation of Cash, Cash Equivalents, and Restricted Cash) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Cash and cash equivalents	$ 323,861	$ 584,581
Restricted cash and cash equivalents included in other current assets	5,086	2,697
Restricted cash and cash equivalents included in other assets	4,457	4,451
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	$ 333,404	$ 591,729	$ 330,007	$ 310,937