Summary of Significant Accounting Policies (Reconciliation of Cash, Cash Equivalents, and Restricted Cash) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 281,986	$ 301,353
Restricted cash and cash equivalents included in current restricted assets	2,832	9,081
Restricted cash and cash equivalents included in non-current restricted assets	4,485	503
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	$ 289,303	$ 310,937	$ 362,713	$ 398,608