Basis of Presentation (Cash and Cash Equivalents and Restricted Cash) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Cash and cash equivalents	$ 402,672	$ 573,210
Restricted cash included in other current assets	970	938
Total cash and cash equivalents and restricted cash at end of period	$ 403,642	$ 574,148	$ 443,414	$ 488,127