Quarterly report pursuant to Section 13 or 15(d)

Basis of Presentation (Cash and Cash Equivalents and Restricted Cash) (Details)

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Basis of Presentation (Cash and Cash Equivalents and Restricted Cash) (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]        
Cash and cash equivalents $ 767,873 $ 573,210    
Restricted cash included in other current assets 974 938    
Total cash and cash equivalents and restricted cash at end of period $ 768,847 $ 574,148 $ 485,639 $ 488,127