Supplemental Disclosures to Consolidated Statements of Cash Flows (Cash, Cash Equivalents and Restricted Cash) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Cash Flow Elements [Abstract]
Cash and cash equivalents	$ 491,257	$ 573,210	$ 487,163
Restricted cash included in other current assets	775	938	964
Total cash and cash equivalents and restricted cash at end of period	$ 492,032	$ 574,148	$ 488,127	$ 2,001,481