Summary of Significant Accounting Policies (Reconciliation of Cash, Cash Equivalents, and Restricted Cash) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Cash and cash equivalents	$ 312,638	$ 584,581
Restricted cash and cash equivalents included in current restricted assets	3,719	2,697
Restricted cash and cash equivalents included in non-current restricted assets	4,454	4,451
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	$ 320,811	$ 591,729	$ 289,303	$ 310,937