Summary of Significant Accounting Policies (Components of Cash, cash Equivalents and Restricted Cash) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cash and cash equivalents	$ 584,581	$ 301,353	$ 393,530
Restricted cash and cash equivalents included in current restricted assets	2,697	9,081	914
Restricted cash and cash equivalents included in non-current restricted assets	4,451	503	4,134
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$ 591,729	$ 310,937	$ 398,578	$ 255,187